You cannot see dissolved CO2, but you can estimate it from two numbers you already test. This aquarium CO2 calculator takes your pH and carbonate hardness (KH) and returns the CO2 in parts per million, then tells you whether it sits in the planted-tank sweet spot or the danger zone.
Aquarium CO2 Calculator
Enter your tank's pH and carbonate hardness (KH) to estimate the dissolved CO2 in parts per million, and see whether it sits in the planted-tank sweet spot.
Estimate uses the standard relationship CO2 (ppm) = 3 × KH × 10^(7 − pH). It assumes your pH is driven by carbonate hardness alone. Phosphate buffers, tannins, or pH-altering additives throw the reading off, so treat it as a guide and watch your fish and a drop checker. The planted target is 20 to 30 ppm; above about 35 ppm fish start gasping at the surface.
The pH and KH relationship
In water buffered by carbonate hardness, CO2, pH and KH move together, so a known pH and KH pin down the CO2. The estimate assumes your pH is driven by KH alone. Phosphate buffers, tannins from driftwood, or pH-altering additives skew it, so treat the number as a guide and back it up with a drop checker. For the full picture on these parameters, read pH, KH and GH explained.
The safe range
Planted tanks aim for 20 to 30 ppm, enough to drive growth and pearling. Above about 35 ppm fish start gasping at the surface, and the risk peaks overnight when plants stop consuming CO2 and it builds up. If fish crowd the surface in the morning, cut the injection rate and add surface agitation.
Do you even need CO2?
Only high-light, heavily planted tanks need pressurized CO2. A low-tech tank with hardy plants and a modest light grows fine without it. Match your light to the plants with the lighting calculator, and see every tool on the aquarium calculators hub.
Frequently asked questions
How do I measure CO2 in my aquarium?
Estimate it from pH and KH with the formula CO2 = 3 x KH x 10^(7 – pH), which this calculator runs for you, then confirm with a drop checker.
What is a safe CO2 level?
20 to 30 ppm for a planted tank. Above 35 ppm risks the fish.
Why do my fish gasp in the morning?
CO2 builds up overnight when plants stop using it. Reduce injection and increase surface movement after dark.