Aquarium CO2 Calculator
Calculate dissolved CO2 in your planted tank from pH and KH readings. Instantly check if CO2 levels are safe for fish and optimal for plant growth — no sign-up needed.
CO2 Calculator
Measure with a reliable test kit or calibrated probe
Degrees of carbonate hardness (dKH)
Enter your values above to see results
Equipment for better CO2 control
Essential tools for measuring and monitoring CO2 in a planted tank.
Visual CO2 monitor — turns green at the optimal level, yellow if too high, blue if too low
Measures carbonate hardness — the other half of the CO2 formula alongside pH
Fast, accurate pH readings — more reliable than liquid tests for daily CO2 monitoring
I’ve used everything listed in my own tanks. If you buy through a link, I earn a small commission — at no extra cost to you, and it never affects what I recommend.
Related tools
Frequently Asked Questions
- What is the ideal CO2 level for a planted aquarium?
- The ideal range is 20–30 ppm for most planted tanks. At this level, plants have enough carbon for vigorous photosynthesis and healthy growth without posing a risk to fish. Below 15 ppm, many plants struggle — especially demanding species like stem plants and carpeting plants. Above 30 ppm, fish begin to experience respiratory stress.
- How do I measure CO2 in my aquarium?
- The most affordable method is a drop checker: a small glass vessel filled with indicator solution that turns yellow (high CO2), green (optimal), or blue (low CO2). For more precision, use a calibrated pH probe and KH test — then this calculator gives you a numerical result. Direct dissolved CO2 probes exist but are expensive and mainly used in large installations.
- Can high CO2 kill fish?
- Yes. Fish breathe by exchanging gases across their gills — they expel CO2 and absorb oxygen. When dissolved CO2 exceeds about 30–35 ppm, fish begin to show signs of stress: gasping at the surface, lethargy, and loss of colour. Above 40 ppm, CO2 becomes acutely toxic. If you see fish gasping, increase surface agitation immediately and check your CO2 system.
- Why does KH affect CO2 levels?
- KH (carbonate hardness) is a measure of the bicarbonate buffering capacity in your water. Bicarbonates and CO2 exist in chemical equilibrium: as CO2 rises, pH drops, and vice versa. Because this relationship is mathematically predictable, measuring pH and KH together lets you calculate dissolved CO2 without a direct measurement. Higher KH means the water can hold more CO2 at a given pH.
- My CO2 reads low but I'm injecting — what's wrong?
- Common causes: the diffuser is placed too close to the outlet and CO2 is being gassed off before dissolving; surface agitation is too high, stripping CO2 from the water; the bubble rate is too low for your tank volume; or the CO2 is only running during the day and your reading was taken at night. Try placing the diffuser in a low-flow area, reducing surface movement, and increasing your bubble count gradually.