Aquarium CO₂ calculator from KH and pH
Enter the KH and pH of your tank water, or how much the pH dropped against a degassed sample. The result is an estimate in mg/l (ppm).
A common target for a planted tank with CO₂.
How much CO₂ is right
Planted tanks with injected CO₂ usually aim for 20–30 mg/l. Below 15 mg/l demanding plants struggle; above 35 mg/l watch your fish – if they gasp at the surface, cut CO₂ immediately and increase surface agitation.
When the KH/pH formula fails
The formula assumes KH comes only from carbonates. Phosphate buffers, humic substances from roots and leaves or aquasoil distort it, often well upward. The more reliable method is the second one: measure the pH of a sample left to stand for a day (degassed) and the tank pH at the end of the photoperiod. A drop of 1.0 corresponds to roughly 30 mg/l CO₂.
A drop checker with 4 dKH reference solution shows the same thing with about a two-hour delay: green means roughly 30 mg/l.
Guides
- Cycling a planted aquarium: the first weeks and when to add fishHow long cycling a planted tank takes, what to test, how often to change water with aquasoil and when it is safe to add shrimp and fish.
- Aquarium water changes: how often and how muchHow much water to change in a planted aquarium, how often, what to watch with temperature and chlorine and how to combine it with trimming and cleaning.
- Algae in a new aquarium: which are normal and what to doBrown diatoms, hair algae and green dust in a new planted aquarium. Why they appear, which go away on their own and what helps without chemicals.
- Fertilizing a planted aquarium: macros, micros and target levelsWhat aquarium plants need, guideline levels for NO₃, PO₄, K and Fe, the difference between EI and lean dosing and how to spot a deficiency.