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:

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 CO₂ gets very murky and impossible to predict.

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 CO₂, UNLESS 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.

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.

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.

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 CO₂ raises 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.

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.

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.

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.

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.

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