Aquarium CO2 Calculator (pH / KH)
Enter your tank pH and carbonate hardness (KH) to estimate the dissolved CO2 in a planted aquarium, see whether you are in the ideal green zone for plants, and catch levels that put your fish at risk.
KH is carbonate hardness in degrees (dKH). A typical liquid test kit reports it directly. 4 dKH is a common planted-tank value.
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ppm CO2
Reality check: this estimate assumes CO2 is the only thing setting your pH. Tannins from driftwood, phosphate buffers, peat, and active substrates all shift pH on their own and make the number unreliable. A drop checker filled with a 4 dKH reference solution is the trustworthy real-world test. Aim for a lime-green reading.
Gear to dial in your CO2
Pressurized CO2 plus a drop checker is the reliable way to hold the green zone. Sizes below are starting points.
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How the pH and KH CO2 Estimate Works
Carbon dioxide dissolved in water forms a small amount of carbonic acid, and that acid lowers your pH in a way that depends on how much carbonate hardness (KH) is present to buffer it. Because of that relationship, you can work backward from a pH and KH reading to estimate dissolved CO2. The standard hobby formula is CO2 in ppm equals three times your KH in dKH, multiplied by ten raised to the power of seven minus your pH. Enter both numbers above and the calculator does the math, rounds to one decimal place, and tells you which band you land in.
As an example, a tank at pH 6.8 with a KH of 4 dKH comes out to about 19 ppm of CO2, which sits comfortably in the green zone for plants. Drop the pH to 6.6 at the same KH and CO2 climbs to roughly 30 ppm, right at the top of the ideal range. That sensitivity is exactly why small CO2 adjustments matter so much, and why you should change your bubble rate slowly and observe the tank between changes.
Reading the Result Bands
Below about 15 ppm your plants are carbon-limited and will grow slowly, so there is room to add more if you want faster, lusher growth. The 15 to 30 ppm window is the green zone that most planted-tank keepers target, giving plants ample carbon while keeping fish comfortable. From 30 to 40 ppm you are running rich, so watch your fish for surface gasping and keep oxygen high. Above 40 ppm is genuinely dangerous and you should reduce CO2 right away, increase surface agitation, and add an airstone until levels come down.
Why the Drop Checker Is the Real Test
The pH and KH formula only holds true when carbonic acid from CO2 is the single thing moving your pH. The moment anything else gets involved, the estimate drifts. Driftwood releases tannins, some buffers and pH-down products add phosphate or other acids, peat softens and acidifies, and active aquasoil substrates leach buffering compounds for months. Any of these will make the calculated CO2 wrong, sometimes badly so. That is why a drop checker is the gold standard. Fill it with a known 4 dKH reference solution and bromothymol blue indicator, hang it in the tank, and read the color in the afternoon: blue is too low, lime green is your target, and yellow means back off before your fish suffer.
Balancing CO2 With Light and Nutrients
CO2 does not work in isolation. Plants need light, carbon, and nutrients together, and the highest demand for CO2 comes when your lighting is strong. If you push light hard but starve the tank of CO2, you invite algae and stressed plants. The reverse is also true: rich CO2 with weak light is wasted. Match your CO2 to your lighting intensity, run it only during the photoperiod, and feed your plants with a sensible fertilizer routine. Our lighting calculator helps you gauge whether your fixture is low, medium, or high light, and the fertilizer dosing calculator sets your nutrient schedule. Keep up consistent water changes and the whole system stays in balance.
Keep going: dial in the rest of your planted tank.
Frequently Asked Questions
What is the ideal CO2 level for a planted tank?
Most high-light planted tanks aim for roughly 25 to 30 ppm of dissolved CO2 during the photoperiod, which lands in the green zone on a drop checker. That level gives plants plenty of carbon to grow without pushing fish into distress. Lower-tech or low-light tanks can thrive on far less, since the plants are not demanding much carbon. Always raise CO2 slowly over days and watch your fish closely as you approach 30 ppm.
What are the signs of too much CO2 in my aquarium?
The clearest warning sign is fish gathering at the surface and gasping or breathing rapidly, because high CO2 makes it hard for them to offload their own carbon dioxide. You may also see fish hanging near the filter outflow where oxygen is highest, or becoming lethargic and losing color. If you notice any of this, turn the CO2 off immediately, increase surface agitation, and add an airstone. Then dial your CO2 back down before trying again.
What drop checker color means my CO2 is right?
A drop checker filled with 4 dKH reference solution and bromothymol blue indicator turns lime green at roughly 30 ppm, which is the sweet spot for planted tanks. Blue means your CO2 is too low and plants want more. Yellow means CO2 is too high and your fish are at risk. Because the indicator lags by an hour or two, read it in the afternoon, not first thing after the CO2 turns on, for an accurate picture.
Why should I turn CO2 off at night?
Plants only use CO2 for photosynthesis when the lights are on, so injecting it in the dark just builds up dissolved CO2 with no benefit and starves your fish of oxygen overnight. Put your CO2 on a timer or solenoid that shuts off about an hour before lights-out and turns on about an hour before lights-on. This keeps levels in the green zone during the photoperiod and lets oxygen recover while the tank is dark.
Why is this pH and KH estimate sometimes inaccurate?
The pH and KH method assumes carbonic acid from CO2 is the only thing changing your pH. In the real world, tannins from driftwood, phosphate buffers in some products, peat, active substrates like aquasoil, and other acids all shift pH independently of CO2. When that happens the formula overestimates or underestimates your true CO2. Treat the number as a starting reference and confirm with a drop checker using a 4 dKH reference solution.
Do I really need pressurized CO2 to grow plants?
No. Plenty of low-tech tanks grow hardy plants like anubias, java fern, cryptocoryne, and stem plants with no added CO2 at all, relying on what fish and bacteria produce. Pressurized CO2 mainly unlocks demanding carpet plants, fast growth, and the most vivid colors in high-light setups. If you are not chasing those, you can skip it and simply choose plants suited to a no-CO2 environment.
How do I raise CO2 safely without harming fish?
Increase your bubble rate in small steps, no more than a tiny adjustment every few days, and watch the drop checker and your fish after each change. Keep good surface movement so oxygen stays high even as CO2 rises, and never chase the green zone overnight. If fish start gasping at the surface, you have gone too far, so back the CO2 off and let the tank settle before trying a smaller increase.