Estimative Index

15.5.3. Estimative Index

The Estimative Index (EI) method is the most popular method of dosing fertilizers in high-tech planted CO2-injected aquariums.  I find it to be somewhat complicated and very time-consuming to implement. But there is no question it will give one a very beautiful high-tech planted aquarium. Most of the data on the Estimative Index will be found on Dr. Tom Barr’s blog: The Barr Report.

The Meat of the EI system

The EI system is defined by the fertilizer addition schedule. The schedule requires adding two different water-based fertilizer solutions on alternative days for six days of the week:

  • Day 1 – do a 50% to 75% water change, add no fertilizer
  • Day 2 –  add iron and micronutrients (micro) solution
  • Day 3 –  add NPK fertilizer (macro) solution
  • Day 4 –  add iron and micronutrients (micro) solution
  • Day 5 – add NPK fertilizer (macro) solution
  • Day 6 – add iron and micronutrients (micro) solution
  • Day 7 – add NPK fertilizer (macro) solution

All these additions are typically very heavy applications of both macro and micro fertilizers. This is so that plant growth is never limited by the fertilizer level.

High Tech planted aquarium
High Tech planted aquarium

Estimative Index Macro Formula

If one searches the web for formulas for the macro portion of the EI fertilizer, one will find the formulas vary considerably from one user to another. For our purposes, we will use a 3-0.6-11 NPK macro liquid fertilizer.

To use the EI methodology , mix up one liter of hot distilled water with the following:

  • 328 grams potassium nitrate (NPK 13-0-44)
  • 16 grams mono-potassium di-hydrogen phosphate (NPK 0-52-34)

First, dissolve the potassium phosphate into the hot water and then dissolve the potassium nitrate. This creates roughly one liter of water-based fertilizer solution with a composition of roughly 3-0.6-11 NPK. For a well planted high tech tank add one milliliter of this solution per ten gallons three times a week. 

Note that phosphates form a compound called “polyphosphate” which can form a filmy cloudy kind of mulm or gel at the bottom in a bottle of fertilizer. Just ignore it. Do not store the fertilizer solution in the refrigerator as the chemicals will crystalize out of solution.

As an alternative to making your own solution on can use something like Aquarium Co-op Easy Green. It just gets very expensive, especially for larger tanks.

High Tech Planted Aquarium
High Tech Planted Aquarium

Iron plus Micronutrient Micro Solution

There are not a lot of variations of the micro formula. For a micro fertilizer formula mix up 750 milliliters of distilled water and 250 milliliters of vinegar with 20.5 grams of iron sulfate (ferrous sulfate heptahydrate which is 20% iron by weight) and 5 grams potassium sorbate preservative. It is particularly important to store this iron fertilizer solution in the refrigerator to prevent mold from growing in it, as mold can feed on diluted vinegar.

One can also add 4 to 8 grams of garden store micronutrient mix to the vinegar. The use of micronutrients other than iron is somewhat controversial.  I can find no research that says the micronutrients (other than iron) are necessary with a normal water supply. Nor can I find any research that says a moderate amount of micronutrients (other than iron) do any damage.

Add one milliliter of this iron (plus micronutrients if desired) water-based fertilizer solution per 10 gallons three times a week for a moderately planted aquarium with moderate CO2 and moderate lighting. Note that one can increase the iron concentration and simply proportionately reduce the amount put in. For instance, if one has a moderate 50-gallon tank one can take the ferrous sulfate addition to 60 grams in a liter (12×5=60) and then simply add one millimeter three times a week.

And alternative is to simply use a fertilizer like Aquarium Co-op Easy Iron. It just gets kind of expensive, especially if you have large tanks.

Planted Aquarium
Planted Aquarium

More in depth Analysis of the EI system

What follows is a very boring, long and complex analysis that is only for real nerds, like the author.

The Formula for the Macro EI Fertilizer

The EI index has been around a long time and many of the original articles are very difficult to find on the internet. So it is difficult to “find the meat” as to the macro fertilizer composition, especially if one does not want to use dry fertilizers.

We will herein use the “NPK” fertilizer identification system which identifies the ratio of three chemical species in the fertilizer, nitrogen as simply N, phosphorus as P2O5, and potassium as K2O.

If one spends a few hours leafing through the Barr report, the NPK ratio of the Estimative Index (“EI”) fertilizer system appears to vary according to the type of planted aquarium being treated.  The optimum “NPK” ratio for the Estimative Index Fertilizer for a very low tech beginner planted aquarium appears to be roughly five parts nitrogen (N) to one part phosphorus (P2O5) to fifteen parts potassium (K2O), or 5-1-15 NPK. The optimum “NPK” ratio for high tech tanks appears to be about three parts nitrogen to two parts phosphorus to fifteen parts potassium or 3-2-15 NPK.

Both ratios are very loose guidelines. Many very successful planted aquarium hobbyists use the macro additions of the estimative index with fertilizer ratios and quantities much different than the “guidelines”. This means things can get very confusing if one researches the Estimative Index on the web,

High Tech Planted Aquarium
High Tech Planted Aquarium


A search of the Barr report and the applicable YouTube channels and doing some pretty complicated math will lead one to understand that the recipe for the fertilizer generally varies depending on the type of aquarium. Nitrogen seems to be more important in low tech tanks with easy “green” plants. Nitrogen seems to be decidedly less important with high tech tanks with difficult, multicolored plants. These are the resulting liquid recipes:

For a low tech tank mix up one liter of hot distilled water with the following:

  • 328 grams potassium nitrate (NPK 13-0-44)
  • 16 grams mono-potassium di-hydrogen phosphate (NPK 0-52-34)

This gives an NPK of 3-0.6-11 and is the formula given above. This formula is fine for ALL scenarios. But some enthusiasts modify the formula to the aquarium. If one wants to do that the two following formulas are useful.

For a moderate tech tank mix up one liter of hot distilled water with the following:

  • 290 grams potassium nitrate (NPK 13-0-44)
  • 35 grams mono-potassium di-hydrogen phosphate (NPK 0-52-34)
  • 30 grams potassium sulfate (NPK of 0-0-55)

This gives an NPK of 2.7-1.2-11, a little lower on the nitrogen and higher on the phosphate.

For a high tech tank mix up one liter of hot distilled water with the following:

  • 250 grams potassium nitrate (NPK 13-0-44)
  • 55 grams mono-potassium di-hydrogen phosphate (NPK 0-52-34)
  • 55 grams potassium sulfate (NPK of 0-0-55)(tough to dissolve)

This gives an NPK of 2.3-2-11, again, a little lower on the nitrogen and higher on the phosphate.

For the potassium sulfate additions put the sulfate in first as it can be stubborn to dissolve.

High Tech Planted Aquarium
High Tech Planted Aquarium

Other “Variations”

These are not “rules” or even good “guidelines”. For instance, some “Dutch Style” very high tech planted aquarium enthusiasts may double or even triple the volume of the additions above and go with a very high phosphate/low nitrate formula of an NPK of 0.07-1.3-4.5:

To make up this formula mix up two liters of hot distilled water with the following:

  • 11 grams potassium nitrate (NPK 13-0-44)
  • 50 grams mono-potassium di-hydrogen phosphate (NPK 0-52-34)
  • 120 grams potassium sulfate (NPK of 0-0-55)(tough to dissolve)

This results in a formula whose dosing needs to be doubled as it is two liters and only half strength of the others. This low nitrogen formula seems to give the best color and the most compact growth of tough to grow red and yellow colored plants, especially when coupled with a severe cut-back on the micros (iron). Technically this variation is “limiting” in that the amount of nitrogen and iron are less than the plants can utilize. This lack of nitrogen and iron bleaches the green from new leaves and makes them vibrant red, purple or yellow.

The fact that the potassium and phosphorus additions help the plants even without a lot of nitrogen (and iron) sort of negates what is called the “barrel stave” analogy but so be it. But note getting to the point where the plants get enough nitrogen to grow but not enough to color the leaves green, is a very fine line and tricky, to put it mildly. The point is that this high degree of variation of the “recipe” makes it very confusing for a beginner reading the comments in a blog.

High Tech Planted Aquarium
High Tech Planted Aquarium


This fertilizer formulation is flexible. If one sees new leaves with yellowing and the nitrate level of the water is below 10, add potassium nitrate. If one has holes in the leaves go ahead and add some potassium sulfate. If older leaves are turning yellow, add potassium phosphate. Do not be afraid to experiment. Different plants respond differently to fertilizers and water chemistry plays a big factor.

Note CO2 is far more important than the fertilizer formula, as is lighting. So only change the fertilizer formula if one sees clear signs of a deficiency. And do NOT overthink this. I cringe when I hear how some hobbyist tried to control a specific species of algae by changing this or that in his fertilizer. This is rarely productive but leads to a huge number of myths where “correlation does not imply causation”.

For instance, one hobbyist had his green spot algae disappear when he reduced the nitrogen. The green spot algae probably disappeared because his plants began to thrive but the nitrogen reduction gets the credit. And a myth is born: “To control green spot algae, reduce the nitrogen“.

High Tech Planted Aquarium
High Tech Planted Aquarium

The Estimative Index in Depth

The scientist that came up with the EI method, Dr. Tom Barr,  says it is about “90% based” on an older system that was originated by Sears and Conlin (

Back in the 80s, there was a rather expensive commercial fertilizer program called the Dupla fertilizer system. While the Dupla fertilizer did not have a published composition, Sears and Conlin guessed at the composition. Sears and Conlin then used cheap chemicals to “duplicate” what they surmised to be the Dupla system. They called it “Poor Man’s Dupla Drops” or “PMDD”. It had large amounts of potassium, much smaller amounts of nitrogen, and NO phosphate. It also had large amounts of magnesium and micronutrients.

Dr. Barr says he just made some “tweaks” to the PMDD system and called it the “Estimative Index”. Dr. Barr is being very modest. His formulation is MUCH different than the PMDD fertilizer formulation (see comparison below). In addition, his Barr Report website makes a whole series of recommendations on planted aquariums and fertilizers that go way beyond simple water-based fertilizer solution recipes. ALL the recommendations on the Barr Report website are excellent.

Planted Discus Aquarium
Planted Discus Aquarium

Dr. Barr is a marine biologist that kept high-tech planted aquariums professionally since 1996. And he “knows his stuff”, to put it mildly. Almost every point he makes checks out in research papers. His blog has a link to a source of the chemicals needed to make the fertilizer. But Dr. Barr doesn’t push this link and he doesn’t seem to be associated with it.

But, unfortunately, Dr. Barr only has a blog that is a back and forth of various comments. The blog is the “Barr Report” ( site now largely edited by the very knowledgeable Jason King).  I wish Dr. Barr had written a book or something. I’d pay a lot for a copy! This blog and its forum are an incredible gold mine on how to properly fertilize a planted aquarium.

But I found it very difficult to “find the meat”, requiring paging through page after page of comments.  I’ll try to explain it here. At its core the Estimative Index is simple:

But understanding why this works is somewhat difficult. Note that we did not personally use the EI methodology in the past simply because it is very labor intensive, requiring frequent large-scale water changes. I’m just too lazy for all that! But I’m now very successfully using this system in a tank with drip water changes and fertilizer additions with mini-pumps.

Note also that, as with most blanket statements, there are exceptions. Sometimes the Estimative Index does NOT “provide non-limiting nutrients”. The ability of some plants to absorb some nutrients is the stuff of legends. Just look up Vallisneria and potassium absorption some day.

High Tech Planted Aquarium
High Tech Planted Aquarium

An Example: How it Works

Let us look at a high-tech planted aquarium example to understand the EI method. Let us look at an aquarium with a large mass of healthy rooted stem plants in the Fluval stratum. It has a 30 ppm CO2 injection system and a high-tech powerful LED light 14 hours a day. It has five varieties of stem plants.

Say we are not using the Estimative Index. Say we are dosing once a week with a complete water-based fertilizer solution like a 4-1-8 NPK at 2 ppm nitrogen, 0.5 ppm phosphate, and 4 ppm potassium per week with an added boost of 0.25 ppm trace (iron and micros) added. Let us look at four different “scenarios”:

  • Because we have a large mass of plants with lots of CO2 and light, the amount of nutrients we are supplying is too low. All five of our plant varieties are not “thriving”. Stem plants that are not thriving are not putting out chemicals that control algae (negative allelopathy) or they might be not absorbing the nutrients from the water around the plant. And algae do not need high nutrients. So the plants become coated with algae and they die.
  • An alternative scenario is where nitrogen, phosphorus, and iron levels are fine.  But one of the plants is using up all the potassium very rapidly (different plant species absorb nutrients differently). So four of the plants are showing signs of potassium deficiency (dead spots in the leaves) and green spot algae are growing on their leaves as green spot algae don’t need potassium and the plants are not healthy.
  • A third scenario is that for whatever reason one of the nutrients has built up to high levels while the other nutrient is at a low level. When nutrients in a tank get “out of balance” algae tend to thrive. So an aquarium with a 5 ppm phosphorus level but only 1 ppm nitrogen level will, theoretically, not be “balanced”, so algae can grow and kill the plants.
  • Because we are adding the iron at the same times as the phosphate, the two are tying each other up and the plants are suffering because of it.
High Tech planted aquarium
High Tech planted aquarium

Let us say we now use the EI system of a water-based fertilizer solution on this aquarium

  • Day 1 – do a 50% to 75% water change, add no fertilizer
  • Day 2 –  add 0.2 ppm iron and micronutrients solution
  • Day 3 –  add 3 ppm nitrogen, 0.6 ppm phosphorus (P2O5), and 11 ppm potassium (K2O) fertilizer solution (3-0.6-11 NPK)
  • Day 4 –  add 0.2 ppm iron and micronutrients solution
  • Day 5 –  add 3 ppm nitrogen, 0.6 ppm phosphorus (P2O5), and 11 ppm potassium (K2O) fertilizer solution (3-0.6-11 NPK)
  • Day 6 – add 0.2 ppm iron and micronutrients solution
  • Day 7 –  add 3 ppm nitrogen, 0.6 ppm phosphorus (P2O5), and 11 ppm potassium (K2O) fertilizer solution (3-0.6-11 NPK)

This is NINE times the level of fertilization of the three scenarios where we have algae. And the fertilization is very frequent, which ensures all the plants can get their share. Frequent large water changes will ensure nothing gets out of balance. So the plants in all three scenarios will thrive and kill off the algae.

planted aquarium
planted aquarium

EI in More Depth

Dr. Barr very deliberately called his method the “Estimative Index”. The terms are very important per Dr. Barr. It is an “Index”. And indexes go up and down. And it is “Estimative”, i.e. it requires some estimations of the mass of plants and their particular fertilizer needs.

For instance: If the CO2 and lighting are reduced to half the level above, the aquarium will do just fine with half the level of water-based fertilizer solution. Adding the amount above is just wasting chemicals. One needs to “index” the amounts according to three variables, plant mass, CO2, and lighting. Lower plant masses should have lower fertilization, lower lighting, and lower CO2 levels. CO2 levels are especially important.

A useful chart for high-tech planted aquariums using the EI methodology and the water-based fertilizer solution formulas found above is this:

EIAmount Added 3 times a Week
Gallons/LitersHigh Tech Few PlantsHigh Tech Some PlantsHigh Tech Many Plants
10 / 386 drops1 milliliter3 milliliters
25 / 950.8 milliliter2.5 milliliters7.5 milliliters
50 / 1901.6 milliliter5 milliliters15 milliliters
75 / 2832.5 milliliters7.5 milliliters22 milliliters
100 / 3803.3 milliliters10 milliliters30 milliliters
EI High Tech Aquarium Fertilizing Program

This chart is the key to the Estimative Index. It is the macro and micro dosing for the aquariums. It is typically a very heavy load of fertilizer.

Dr. Barr recommends only 5% to 10% of the above fertilization rates for low-tech setups with no CO2 injection and low light. If Dr. Barr recommends it one can bank on it!  If the tank only has starter plants that have not built up masses of foliage one can use 10% to 20% of the above. After three months one might have climbed to 50% of the above levels. This is NOT a rigid program. It ends up being something along these lines:

MaturityOne WeekSix MonthsOne Year
Plant mass50 grams500 grams1,000 grams
Nitrogen per week0.3 ppm3.4 ppm7 ppm
Phosphorus per week0.1 ppm1 ppm2 ppm
Potassium per week1 ppm12 ppm24 ppm
Co210 ppm20 ppm30 ppm
A Balanced Aquarium Fertilizing Program

Each day (save one) water-based fertilizer solutions are dosed, and the nutrients are absorbed by the plants. A key word in this method is the word “Estimative”.  One looks at the mass of the plants, the CO2 levels, and the lighting and ESTIMATES the level of fertilizer needed. At the end of the week, one performs a 50% to 75% water change to “reset” the nutrient concentration in the entire system. And then the entire dosing regime is repeated.

Note all water-based fertilizer solution additions should always be made in the morning when the CO2 comes on. CO2 injection drops the pH and almost all nutrients are better absorbed at lower pHs.

By alternating iron and micronutrients with phosphate additions in water-based fertilizer solutions, one reduces the likelihood of the iron being tied up in highly insoluble iron phosphate.

High Tech Planted Aquarium
high tech planted aquarium

The hobbyists can do larger (which will afford more accuracy) or smaller water change routines, the 50% to 75% is just a guideline. Note some caution is advised with large water changes and water from a well or bore. Well water often has no oxygen in it and can suffocate fish with a large water change. And of course water from a municipal system can have a chlorine pulse.

So if you dosed 10 ppm of NO3, the most it would ever climb to is 20 ppm as long as you did 50% weekly and no uptake took place. If you did a 90% water change weekly, then the most it would build up would be about 12 ppm. More work and effort to do the 90% vs 50%, but you get much smaller ranges, If you chose 25%, now you are at about 40 ppm, so the difference in the percentage of water changes can be used to change the build-up and accumulation.

My recommendation for an EI system is to do a 75% water change. Large scale water changes do NOT hurt fish (that is a common myth). And I measure the nitrate occasionally before adding the fertilizer and I aim for 40 ppm nitrate.

High Tech Planted Aquarium
High Tech Planted Aquarium

Note there are several ways to do this regimen. One can make additions based on the volume of the plants. You can also just measure a parameter such as the nitrate level and adjust the dosing to achieve a certain level of the nutrient. A very good method is to measure the nitrate level just before the water change. Aim for 10, 20, 40 or 80 ppm residual nitrate in the water. If the nitrate is lower than desired level, adjust the fertilizer amount upward. If it is over the desired level, adjust the fertilizer level down.

One may dose dry powdered fertilizers or choose to make a stock solution of water-based fertilizer solution. This is a choice, if one is really into it and one feeds fish daily, then dosing the liquids work pretty well. An attractive alternative is something reefers use. They are called “dosing pumps” and are programmable pumps that can be programmed to add small amounts of liquid on any schedule.

High Tech Planted Aquarium
High Tech Planted Aquarium

Several companies supply dry fertilizer chemical “packages” at VERY reasonable prices. For instance, the NilocG EI Based NPK + CSM+B Fertilizer is designed to be used with the EI (Estimative Index) dosing method. The fertilizer package includes 2lb KNO3, 1/2lb KH2PO4, 1/2lb K2SO4, and 1/2lb CSM+B (Iron, Boron, and micronutrients). This costs less than $20, which is a very reasonable price. It will last a very long time.

GreenLeaf Aquariums (GLA) has a series of fertilizer packages and formulas for measuring out the fertilizers. They are very reasonably priced and their formulas and worksheets are very useful. They supply equal quantities of each fertilizer component in their kits so I recommend only buying the raw chemicals from them.

Note that Dr. Barr has one recommendation that is often overlooked. He recommends that one NOT add a few measly stem plants and expect them to grow out into masses. He recommends one start out with massive amounts of stemmed plants at the very beginning. This is expensive, but it gives the best results.

Planted aquarium
Planted aquarium

Dr. Barr says a weekly cumulative added dose of water-based Estimative Index fertilizer solution nutrients for a high-tech aquarium with high amounts of lighting, CO2, and plants would be around:

  • Nitrate (NO3) range 10-30 ppm
  • Phosphate (PO4) range 1.0-2.0 ppm
  • Potassium (K) range 10-30 ppm

Another way of looking at this is:

  • Nitrogen (N) range 2.2-6.8 ppm
  • Phosphorus (P) range 0.33-0.66 ppm
  • Potassium (K) range 10-30 ppm

Still another way of looking at it, the “NPK” fertilizer analysis, is:

  • Nitrogen (N) range 2.2-6.8 ppm
  • P2O5 range 0.75-1.5 ppm
  • K2O range 12-36 ppm

This last set of numbers corresponds to the NPK analysis method used by most fertilizers. The average of the NPK of the numbers above is 4.4-1.1-24 ppm weekly cumulative dose.

High Tech Planted Aquarium
High Tech Planted Aquarium

Working the weekly numbers above the minimum NPK is 2.2-0.75-12. The maximum is 6.8-1.5-36. But even these highly variable ratios seem to be often violated by those commentators in the Barr report. If you work the numbers on the teaspoon additions below it turns out to be roughly an NPK of 10-13.5-56 ppm per week. If you work the NPK of the daily additions above you get an NPK of 9-1.8-33 ppm per week. This is a very wide variation.

These differences simply reflect the fact that different hobbyists use vastly different ratios of nutrients when using the Estimative Index. Dr. Barr even commented he tried adding 5.5 ppm phosphate per week and got great results.  Because the nutrients are used in such an over abundance, the exact ratio becomes unimportant.

High Tech Planted Aquarium
High Tech Planted Aquarium

So a given aquarium that works very well on a weekly NPK addition of 2.2-0.75-12 might well work equally well with a 2.2-13.5-12 ratio. While it might work better with the higher phosphate, it probably will NOT work any worse. Dr. Barr was adamant about this and my research says he is 100% correct.

Now a tank which is NOT doing well on a weekly NPK addition of 2.2-0.75-12 might well give horrible algae with a 2.2-13.5-12 ratio. This is somewhat difficult for me to wrap my head around but it is accurate. If the plants are not thriving for whatever reason all bets are of when it comes to fertilization.

High Tech Planted Aquarium
High Tech Planted Aquarium

But again, this is an “Estimative Index”. And, like all indices, it is a gross generalization where a lot of assumptions are made which may or may not work in any given situation. So any planted tank enthusiast has to learn how to experiment. The first variable to experiment with is the CO2 timing and level. Easily 80% of the problems can be traced to CO2 per Dr. Barr. Then try both timing and amount of lighting. Then try things like aeration and water movement. Note changing the fertilization is rarely productive.

Carbon dioxide levels are VERY important in high-tech aquariums. The aquarium below was where the operator ran out of CO2 and just let the tank go for a while with no CO2. The result was the stopping of active growth by the plants and the start of a huge algae outbreak.

High Tech Planted Aquarium getting Algae Outbreak
High Tech Planted Aquarium getting Algae Outbreak

Note the deep blue color in the drop checker. And the coating of green algae on SOME of the plants. The hobbyist tore the tank down. This was unfortunate as if the CO2 level had simply been taken above 20 ppm the plants would have resumed growth and killed the algae.

Planted Aquarium
Planted Aquarium

The EI methodology utilizes phosphorus and iron additions to the water column. It also uses nitrate as the nitrogen source. These items are different than what we recommend in the articles linked below. But this method decidedly does work and is worthy of a trial by anyone.

Another paragraph in the Barr Report is this:

“But what about algae? EI theory is based on the idea that if plant growth is taken care of, plant mass will out-compete algae. It says that algae presence is not determined by whether nutrients are available or not, but rather whether or not there is sufficient plant mass to compete with algae (not only in terms of nutrients but space, and light). Healthy plants are the best defense against algae.”

This is simply 100% correct. The best way to control algae is with healthy, thriving plants. And there is no denying that the EI method has created some stunning planted aquariums.

Planted Aquarium
Planted Aquarium

The Math for the Main EI Fertilizer

Note this math was originally done based on what was in the Barr report.  Then it had to be modified to take into account some solubility issues. The math for the nitrogen is:

EI Method Calculation of amount of chemical to add to one liter of water for a moderate high-tech 10-gallon/38-liter aquarium for 1 ml per add
Calculation Potassium NitrateAlgebraNumbersResult
ppm Nitrogen desired per week=A~ 2.5
The Nitrogen content of salt=B0.13
number of adds per week=C3
amount of chemicals=X
ppm per 1 milliliter addD=A/C2.5/30.833
one ppm= 0.038 grams/10 gallons(38 liters)
Grams per one milliliter addZ=Dx0.0380.833×0.0380.0317
Grams Nitrogen per 1-liter solutionY=Zx1.33.032.x1.330.0426
Grams chemical per liter waterX=Y/B0.0426/0.13328
Solubility at 25 degrees C. is 383 grams/liter, i.e. it is OK
EI Aquarium Fertilizer Nitrogen Calculations
High Tech Planted Aquarium
High Tech Planted Aquarium

The math for the Phosphate water-based fertilizer solution is:

EI Method Calculation of amount of chemical to add to one liter of water for a moderate high-tech 10-gallon/38-liter aquarium for 1 ml per add
Calculation Potassium PhosphateAlgebraNumbersResult
ppm phosphate desired per week=A~ 0.5
The phosphate content of salt=B0.52
number of adds per week=C3
amount of chemicals=X
ppm per 1 milliliter addD=A/C0.5/30.167
one ppm= 0.038 grams/10 gallons(38 liters)
Grams per one milliliter addZ=Dx0.0380.167×0.0380.0063
Grams phosphate per 1-liter solutionY=Zx1.333.0063.x1.3338.4
Grams chemical per liter waterX=Y/B8.4/0.5216
EI Aquarium Fertilizer Phosphorus Calculations

The potassium amount is a result of the calculations for the nitrogen and phosphorus.

Calculation of Fertilizer Numbers for EI Fertilizer
328 gms KNO3 (13-0-44)325x.13=42.60328x.44=144
16 gms KH2PO4 (0-52-34)016x.52=8.3216x.34=5.4
1 liter is 1,000 gramsX=42.6/1,344X=8.32/1,344X=149/1,344
Total is 1,344 grams3,170.6211.1
This fertilizer thus has an N-P-K of 3 – 0.6 -11
EI Calculations

Note that the amount of potassium nitrate varies widely in the Barr Report, sometimes exceeding the solubility. And the potassium phosphate amount varies by a factor of three in various locations in the Barr Report. And some folks in the Barr Report have substituted di-potassium mono-hydrogen phosphate (K2HPO4) for mon-potassium di-hydrogen phosphate (KH2PO4).

It is not very important. The EI methodology uses a huge over abundance of nutrients so the exact composition of the fertilizer is not very important.

Planted Aquarium
Planted Aquarium

The math here to obtain exactly 0.25 ppm iron is:

EI Method Calculation of amount of chemical to add to one liter of water for a moderate high-tech 10-gallon/38-liter aquarium for 1 ml per add
Calculation Ferrous SulfateAlgebraNumbersResult
ppm iron desired per week=A0.2
iron content of salt=B0.20
number of adds per week=C3
amount of chemicals=X
ppm per 1 milliliter addD=A/C0.2/30.066
one ppm= 0.038 grams/10 gallons(38 liters)
Grams per one milliliter addZ=Dx0.0380.066×0.0380.0025
Grams iron per 1-liter solutionY=Zx1.012.0025.x1.0122.53
Grams chemical per liter waterX=Y/B2.53/0.212.7
EI Aquarium Fertilizer Iron Calculations

One can also use any solution of acid in distilled water with a pH of 5 to 6 to stabilize the ferrous sulfate. This can be weak solutions of hydrochloric acid (“pool acid”) or any other mineral acid. Adding this acid can take the KH of the aquarium to zero but that is not a problem as long as the pH first thing in the morning is above about 6.0. And even then a pH below 6.0 will only be a problem with fish in the tank as nitrites can become quite poisonous at that low of a pH.

Some with very soft water (less than 4 GH) add 60 grams Epsom Salts and 40 grams calcium acetate to the ferrous sulfate solution to give the plants the calcium and the magnesium they require. Note I avoid using calcium chloride here as I’m not a big fan of mixing plants and unnecessary chlorides. Also note that a bag of crushed coral in the filter is ALWAYS a good idea.

Low Tech Planted Aquarium
Low Tech Planted Aquarium

EI Methodology in a Low Tech Planted Aquarium

The EI methodology is also applicable to a low-tech planted tank with low lighting and CO2 only from the atmosphere or substrate. This is the applicable chart for the above water-based fertilizer solution formulations:

EIAmount Added 3 times a Week
GallonsLow Tech Few PlantsLow Tech Some PlantsLow Tech Many Plants
102 drops4 drops0.6 milliliter
253 drops10 drops1.5 milliliter
506 drops1 milliliter3 milliliters
759 drops1.5 milliliter4.5 milliliters
1000.6 milliliter2 milliliters6 milliliters
EI Low Tech Aquarium Fertilizing Program

One caution about large water changes. If one is on a well or uses tap water from a municipal water system that uses wells there CAN be a problem. Well or bore water CAN be very deficient in oxygen. So if one does a large-scale water change with it, it CAN distress or even kill the fish. So well water often needs to be pre-aerated for several hours before adding it to the aquarium.

Then with municipal water supplies there is always the chance of a chlorine pulse. I just recommend five times the recommended quantity of a sodium thiosulfate water conditioner be added at every water change.

High Tech Planted Aquarium
High Tech Planted Aquarium

Algae, Phosphates, and the PMDD Methodology

In 1996 Sear and Conlin set up two planted aquariums and manipulated various nutrient levels.  From this very crude “test” they concluded one could prevent algae growth in an aquarium by “limiting” phosphate levels ( ).

Dr. Barr was fairly insistent that phosphate is not a “limiting” factor and cannot be used to “limit” algae, Dr. Barr spends a fair bit of time trying to dispel what he feels is a myth. Dr. Barr, as always, is correct. It is a myth.

The difficulty here is in the definition of “limiting”. It is true that if one can get the phosphate level down below 0.05 ppm (that is 50 parts per billion) you can start “limiting” MOST algae, i.e. preventing the growth of algae altogether. Surprisingly, SOME vascular plants can grow with as little as 5 parts per billion phosphates. Unfortunately, since most fish food is 1% phosphate, any tank with fish in it will never get below 50 ppb phosphate. So “limiting”, i.e. preventing, algae growth, isn’t possible in any aquarium with fish in it.

High Tech Planted Aquarium
High Tech Planted Aquarium

That being said, all plants, including algae, have a growth rate roughly proportional to available nutrients. So if one has a phosphate level in the water column of say 1 ppm and a phosphate level in the substrate of say 30 ppm, then rooted vascular plants can grow faster than algae. And these faster-growing vascular plants then inhibit the algae growth, probably via negative allelopathy. This is probably why Sears and Conlin’s experiments worked the way they did.

Sears and Conlin put large doses of iron into the water column. Iron removes phosphate from the water column by forming highly insoluble iron phosphates. The lower pH in the substrate prevented the phosphates from being removed from the substrate. So the rooted vascular plants grew well while the algae did not. When one of the tanks had phosphate added to the water at a level greater than the iron could precipitate it out, algae took off. The algae only took off because the nitrogen was inadequate for the vascular plants, creating an imbalance that algae could take advantage of.

Dr. Barr’s approach is to put heavy quantities of phosphates and nitrates into the water column and simply allow the vascular plants to outcompete the algae, something which works quite well IF you have a lot of plants in the tank.

Planted Aquarium
Planted Aquarium

PMDD Aquarium Fertilizer System

The Poor Man’s Dupla Dosing (PMDD) system is not much used anymore. Just for reference sake, this system was basically:

ElementPMDD SystemEI System
PMDD Aquarium Fertilizer program

Note that the PMDD system was designed to be added either when tests showed low nitrogen levels or to the water change water every water change. This means the EI system will typically have VERY roughly three to ten times the nitrogen concentration of the PMDD system.

Soft Water

There is one situation where a fertilizer of sorts is called for in all types of fertilization. If the water is very soft (dGH less than 3 or GH less than 72) then it needs both calcium and magnesium additions for most plants. The easiest way to do this is to simply add one level teaspoon of Epsom salts (magnesium sulfate) and one level teaspoon of Plaster of Paris (calcium sulfate and calcium carbonate) to every ten gallons of water change water. Because of waters of hydration this 1:1 ratio insures an optimum 3 parts calcium to 1 part magnesium in the water.

High Tech Planted Aquarium
High Tech Planted Aquarium

How to Set Up an Easy EI System

I set up a planted tank where I used the EI system fertilizer scheme simply because the aquarium had CO2 injection, a constant drip water change system and a double line fluid dosing system. With this system, I only had to keep the fertilizer reservoirs filled. The drip was set up to do the equivalent of about an 50% water change once a week.  This is a VERY easy way to do a planted aquarium. For information on setting up a drip system go to this link:

18.2. Drip Water Changes

Note this heavily planted aquarium had a single small (5 watt) DC pump running the large under gravel filters. This kept the aeration down. This low aeration was needed as the tank was on CO2 injected to a 25 to 30 ppm level while the lights were on. Its only fish were some large schools of small tetras and rasboras. It had a high intensity LED light on it for 14 hours per day.

High Tech Planted Aquarium
High Tech Planted Aquarium

I slowly increased the fertilizer amount, trying to hold a 20 ppm nitrate level after the addition of the micro fertilizer (24 hours after adding the nitrate). The plants were all doing very well in the gravel with the EI system. This is to be expected as the iron and phosphate additions to the water column negate the need for reducing conditions around the vascular plant roots.

I used an automatic two station dosing pump. The fertilizer doses were automatically put in when the lights are programmed to come on, simply programmed for alternative days. I used a high tech tank (NPK of 2.3-1.8-11.5) EI fertilizer ratio.  This worked out well. I did not use the ammonium based method discussed elsewhere as I am on 8.2 pH rock hard well water.

High Tech planted aquarium
High Tech planted aquarium

Per Tom Barr

Here is the “meat” of the Estimative Index direct from its inventor, Dr. Tom Barr

The Estimative Index (EI) is a straightforward method for providing nutrients for a planted tank. The idea behind EI is simply introducing an excess amount of nutrients within an aquarium, throughout the week. This excess of nutrients floods the water column and feeds the plants. This is an estimative method; measuring specific nutrient uptake rates is not necessary and no test kits are involved. EI provides a surplus of nutrients that helps to prevent plant deficiencies and allows plant growth unhindered. Most algae-related issues are due to plant deficiencies rather than excess nutrient levels (Ammonium/NH4 + is the exception).

You add a slight excess of nutrients to prevent anything from running out, then do a large water change at the end of the week to prevent anything from building up. This allows you to maintain a range of nutrients without ever using a test kit.

The water change generally takes about the same amount of time once you haul out the hoses etc. so the water change so the time and work difference between a 25 % and 50% water change are fairly small.

The process in which this is done is simple. Each day (or 2-3x a week, weekly for low light tanks) fertilizers are dosed, and the nutrients are absorbed by the plants. With this method being estimative, we can dose fertilizers according to general guidelines suited for our particular setup (see below for regime). At the end of the week, one performs a 50% water change to ‘reset’ the nutrient load in the entire system. And then the entire dosing regime is repeated. The hobbyists can do larger (which will afford more accuracy) or smaller water change routines, but 50% is just a guideline.

The primary fertilizers are the macronutrients – Nitrogen (N), Phosphorous (P), Potassium (K), and the micronutrient – Iron and trace elements.

The Estimative Index method works best for high-light and well-planted aquariums. However it is not limited to higher light setups, smaller quantities of fertilizers can be dosed if low light is used. Also, the frequency may be reduced to 1-2x a week at low light(1.5-2w/gal).

General Dosing Guideline for High Light and well-planted aquariums.

10- 20 Gallon Aquariums
1/8 tsp KNO3 (N) 3x a week
1/32 tsp KH2PO4 (P) 3x a week
1/32 tsp K2SO4 (K) 3x a week
1/32 tsp  Iron + Trace Elements 3x a week
50% weekly water change

20-40 Gallon Aquariums
¼ tsp KN03 3x a week
1/16 tsp KH2P04 3x a week
1/16 tsp K2S04 3x a week
1/16 tsp Iron + Trace Elements 3x a week
50% weekly water change

40-60 Gallon Aquariums
1/2 tsp KN03 3x a week
1/8 tsp KH2P04 3x a week
1/8 tsp K2S04 3x a week
1/8  Iron + Trace Elements 3x a week
50% weekly water change

60 – 80 Gallon Aquariums
3/4 tsp KN03 3x a week
3/16 tsp KH2P04 3x a week
1/4 tsp K2S04 3x a week
¼ tsp  Iron + Trace 3x a week
50% weekly water change

100 – 125 Gallon Aquarium
1 1/2 tsp KN03 3x a week 
½ tsp KH2P04 3x a week
½ tsp K2S04 3x a week
½ tsp  Iron + Trace 3x a week
50% weekly water change

EI target ranges
CO2 range 20-30 ppm
NO3 ranges from 5-30 ppm
K ranges from 10-30 ppm
PO4 range 1.0-2.0 ppm
Fe 0.2-0.5 ppm or higher
GH range 3-5 degrees ~ 50ppm or higher”

Tom Barr

These directions came out in 2006. They remain applicable. Note this nutrient dosing and range is for a VERY high-tech aquarium with a large amount of plants, lighting, and CO2. The target dosing and ranges for a low tech aquarium might only be 5% of these targets.

But it must be emphasized that these ranges mean little. Because one is doing such an excess of nutrients the exact levels are of little consequence. So don’t obsess over them.

High Tech Planted Aquarium
High Tech Planted Aquarium

Fertilizers in More Depth

We go into aquarium fertilizers in more depth in the following links:

15.5. Aquarium Fertilizing

15.5.1. Ready-Made Fertilizer

15.5.2. Fertilizer Programs

15.5.4. NH4 + Tabs Fertilizer

15.5.5. DIY Epiphytic Fertilizer

15.5.6 Fish Food as Fertilizer

15.5.7. DIY Fertilization

Startpage Aquariumscience

Source: – David Bogert



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