KH pH CO2 Relationships in the Aquarium

15.6.2. KH pH CO2 Relationships in the Aquarium

The relationship of CO2, pH and dKH levels is called the “Bermuda Triangle” of aquarium chemistry with good reason. It is very complex and making simplistic assumptions about it can easily lead one astray and kill fish.

If one is a real nerd, like the author, read on, just be prepared to get very confused (and very bored!).

CO2/pH/dKH Charts

The following chart is can be used to determine the CO2 level in aquariums:

CO2, pH, dKH chart
CO2, pH, dKH chart

This table relates CO2 content to the pH and dKH levels in the water.  This relationship is more complex than the table lets on but it is a good starting point. Note there is no row for zero dKH. When there is no dKH the relationship of pH and COgets very murky and impossible to predict.

Protomelas taeniolatus Fire Hap
Protomelas taeniolatus Fire Hap

The dKH (also known as KH [dKH x 18], alkalinity and carbonate hardness) represents the pH buffering capacity of the water. Harder water will have a higher buffering capacity and vice versa.

If you measure pH/dKH when the tank is at equilibrium when CO2 injection is off and before the lights have come on in the morning, CO2 levels in the tank will TYPICALLY (but not always) match equilibrium levels with atmosphere – which will be around 2-3 ppm.

Put more clearly, while atmospheric CO2 has a concentration upwards of 400 ppm, the average amount in an open container of pure water stabilizes at around 2-3 ppm, which is called “atmospheric equilibrium”. A tank without CO2 injection will not have elevated levels of CO2UNLESS there is a lot of rotting organic matter in the aquarium OR UNLESS there are a decent number of fish. Rotting organic matter and fish both put carbon dioxide into the water.

Hemichromis elongatus Five Star General
Hemichromis elongatus Five Star General

Myth: A Low KH results in a larger pH swing when adding CO2.

Many people are under the mistaken impression that a low dKH results in large pH swings when adding CO2, while raising the dKH will result in smaller pH swings. This is not the case for any dKH over 1. The dKH will move the start and end pH values, but the pH swing will be the same for a given level of CO2. You can see this in the charts:

  • Example 1: Assume a dKH of 15 degrees and a starting CO2 level of 4.5 ppm, you would have a pH of 8.0. If we then add CO2 to achieve 28 ppm the pH would drop to 7.2, a change of 0.8.
  • Example 2: Assume a dKH of 1.5 degrees and a starting CO2 level of 4.5 ppm, you would have a pH of 7.0. If we then add CO2 to 28 ppm your pH would drop to 6.2, a change of 0.8.

This relationship will break down at extremely low dKH levels (below 1 degree), when there isn’t enough carbonate to completely buffer the acids present. In that case, the pH can drop quickly and dramatically. But if the dKH is 1 degree or higher, then the size of the pH swing when injecting CO2 will be determined only by the amount of CO2 dissolved in the water.

Electric Blue Acara
Electric Blue Acara

Myth: CO2 level can be adjusted simply by adding chemicals to alter the KH or pH.

This is a common misconception when using the CO2/pH/dKH table. It appears that by altering CO2 or dKH values, the other values should ALL move. But this is not true. Treat the pH value you see as a result. If you alter the dKH, then the pH will move. If you alter the CO2 level, then the pH will move. The pH will always react to changes in either of the other two parameters.

Example: The water has a dKH of 3 degrees and a pH of 7.6. If we look at the chart this indicates a CO2 level of 2.3 ppm. One might assume, per the chart, that raising the dKH to 10 degrees the CO2 level would rise to 7.5 ppm. Seems simple enough but it is not correct. If you raise the dKH the pH will rise along with it and the CO2 level will stay at 2.3 ppm.

You cannot alter the dKH levels other than by adding or removing carbonate. You cannot alter the CO2 levels other than by adding or removing CO2.

Maylandia estherae Red Zebra
Maylandia estherae Red Zebra

There is one case I’ve seen where the addition of CO2 resulted in an increase in dKH. This can happen when you have something in the tank that dissolves carbonate into the water. Seashells, crushed coral, and many gravels and rocks will do this. With the addition of CO2, the water turns more acidic, which will increase the dissolving of the minerals. It appears that increasing COraises the dKH, which isn’t really the case. The dissolving minerals raise the KH, and the increase in dKH results in an increase in pH.

In a system using a pH probe and controller to regulate CO2 levels, this can have fatal consequences, since the pH controller will keep trying to lower the pH, but as more CO2 is dissolved, it lowers the pH, which raises the dKH, which raises the pH. So you now have more CO2, but the same pH. So the controller adds even MORE CO2. And it will keep going. So it’s important to know your dKH whenever using pH to judge CO2 levels, especially if you have a form of calcium carbonate in the aquarium.

Mikrogeophagus ramirezi Black Knight Rams
Mikrogeophagus ramirezi Black Knight Rams

The pH-KH-CO2 Relationship and Phosphates

CO2, dKH, and pH have a fixed relationship only if phosphates are not present in significant quantities. Phosphates “buffer” in the range of 5.0 to 7.0 pH. This “buffering” will throw off the CO2/pH/dKH relationship.

There are several sources for phosphates in an aquarium:

  • There are some parts of the country that have high levels of phosphates in their water supply
  • Fish food is typically 1% phosphates
  • Plant fertilizers typically contain significant phosphates
  • Commercial “pH Buffers” which buffer in the 5.0 to 7.0 range typically contain significant phosphates.

For aquariums that have sources of phosphate, determining CO2 levels will be difficult, as the phosphate will throw off the CO2/pH/dKH relationship, which means charts won’t work. Note that the commercially available CO2 test kits which test the water directly (expensive buggers!) will also be invalidated by the phosphates.

OB Peacock
OB Peacock

Tap Water Carbon Dioxide

In some cases, water coming right from the tap can contain very high levels of CO2. This can result in tap water with a low pH. But, in just a few hours, that excess CO2 will dissipate from the water, leaving the normal 3-4 ppm, and the pH will rise. Sometimes, the water might come from the tap with extremely little CO2, which can result in tap water with a high pH. Again, after a few hours, the CO2 level will equalize, and the water will end up with 3-4 ppm CO2 and the pH will drop.

KH CO2 pH Chart
KH CO2 pH Chart

CO2 “Target” for a Planted Aquarium

Per the above chart, a desirable CO2 level is 10-25 ppm (which is indicated in green on the chart). Levels below that don’t provide optimum CO2 concentrations for high plant growth. CO2 concentrations over 30 ppm can be harmful to the fish inhabitants of your tank.

I typically shoot for 15 to 20 ppm CO2. But many run their high-tech tanks at 30 to 35 ppm. While small schools of tetras or rasboras will typically be fine at this CO2 level, a larger fish such as a discus would be in trouble.

Sciaenochromis fryeri OB Blueberry
Sciaenochromis fryeri OB Blueberry

Pulsing Carbon Dioxide

One interesting technique to add CO2 is to maintain a dKH of 2 to 3.  Measure the pH first thing in the morning before the lights turn on in a planted tank. Let’s say one measures 7.4 pH. Add a solenoid to a high-pressure CO2 system which is operated off a pH probe and a timer. Add the CO2 with a reactor that adds the CO2 very rapidly, in a span of less than half an hour. Set the solenoid to open when the lights come on and to turn off at a one-point drop in pH. In this case 6.4 pH.

This adds a single pulse of CO2 to the tank. A single pulse is very safe. A pulse of CO2 to even a level of 40 typically doesn’t kill any fish. My problem is with the electronics. If the solenoid fails to shut off or the pH meter goes bad, one is simply screwed.

Darienheros calobrensis Red Spot Cichlid
Darienheros calobrensis Red Spot Cichlid

Further Information on CO2 Systems

Further information on carbon dioxide in the planted aquarium can be found at these links:

15.6 Carbon Dioxide in a Planted Aquarium

15.6.3. High Tech CO2 System

15.6.6. Measuring CO2

15.6.1 Low Tech CO2 Aquarium System

4.4.3. Carbon Dioxide and pH


Startpage Aquariumscience

Source: Aquariumscience.org – David Bogert

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