Carbon dioxide is the main food for fast-growing aquarium plants, but too much of it harms fish, so planted tank keepers need a way to check the level without a lab. The classic shortcut uses two cheap test results, pH and KH, and a well-known table that links them to dissolved carbon dioxide. This free calculator does that table maths for you in seconds, so you can see whether your level sits low, in range, or too high.
How to use this calculator
Test your tank water with a liquid pH test and a KH test, then enter both numbers and press the button. The tool shows an estimated carbon dioxide level in milligrams per litre, plus a plain reading of what that level means. Test at the same time of day when you compare results, since pH moves through the day as plants use carbon dioxide in the light and release it at night.
How the calculation works
The tool uses the standard formula from the pH and KH table: carbon dioxide equals 3 times KH times 10 to the power of 7 minus pH. For example, pH 7.0 with KH 4 gives 12 milligrams per litre. The formula assumes the pH shift comes mainly from carbon dioxide, so it reads high if your water holds other acids, such as tannins from wood or pH-lowering soils. Treat it as an estimate, not a lab result.
A worked example
A tank at pH 6.8 with KH 5 gives about 24 milligrams per litre, which sits in the usual target range for planted tanks. A tank at pH 7.4 with KH 4 gives about 5 milligrams per litre, which is low for demanding plants. A tank at pH 6.4 with KH 8 gives about 95 milligrams per litre, which is far too high for fish and means the gas injection or the test needs urgent attention before anything else.
Reading your result
Most planted tanks aim for roughly 15 to 30 milligrams per litre during the light hours. Below that, demanding plants grow slowly and algae often moves in. Above 30, watch fish closely: gasping at the surface, fast gill movement, or hanging near the filter outlet all mean too much carbon dioxide or too little oxygen. If fish look stressed, add surface movement and air at once, and only then worry about the plants.
Tips for steady CO2
If you inject gas, run it on a timer that starts an hour or two before the lights and stops an hour before lights out, since plants only use it in light. A drop checker with 4 KH solution is a cheap visual backup that turns green in range and yellow when high. Keep surface movement gentle during the day so gas stays in the water, but add more at night if fish gasp. Change only one setting at a time and wait days before judging.
Common mistakes
The most common mistake is chasing a perfect number while fish gasp, which gets the priority backwards. The second is testing with strips, which are often too vague for this maths, so use liquid tests. The third is forgetting that wood, leaves, and soils also lower pH, which makes the formula over-read carbon dioxide. If the number looks wild, cross-check with a drop checker and watch the fish before you change the gas rate.
Frequently asked questions
Is this calculator a substitute for a drop checker? No, it is a quick estimate, and a drop checker plus fish behaviour together give a safer picture. Do low tech tanks need added carbon dioxide? No, hardy plants grow fine without it, just more slowly. Can carbon dioxide kill fish? Yes, at high levels it displaces oxygen in the blood, so gasping fish mean you should aerate at once and cut the gas, not wait for a retest.
Safety note
This tool is for education and planning only. It does not diagnose fish illness and it cannot replace water tests, a drop checker, or watching your livestock. If fish gasp, hang at the surface, or act oddly, improve aeration first and check ammonia and nitrite too, since several problems look alike. When in doubt, do a water change with matched-temperature water and ask an experienced keeper or vet.
Versión en español: Calculadora de CO2 para acuarios – Nivel según pH y KH.
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