Estimate the dissolved CO2 in a planted tank from your pH and carbonate hardness (KH), and check it against the safe range. Below that, a rough lighting guide.
Formula: CO2 (ppm) ≈ 3 × KH × 10(7 − pH). It assumes your pH is set by carbonate hardness alone. If you dose phosphate buffers, use peat or tannins (blackwater), or have unusual water, the real figure will differ, so trust a drop checker too.
| CO2 (ppm) | Meaning |
|---|---|
| Below 15 | Low. Fine for a low-tech tank; limiting for demanding plants. |
| 15 – 30 | The planted-tank sweet spot. A 4 °dKH drop checker sits lime green around 30. |
| 30 – 40 | High. Great growth, but watch your fish for gasping at the surface. |
| Above 40 | Risky. Enough to stress or suffocate fish. Back the CO2 off. |
Honestly, light is best measured with a PAR meter at the substrate, because a number in lumens tells you little once you account for tank depth and the fixture. This is only a ballpark to place you in low, medium or high light.
Very rough bands: under ~20 lm/L low light (easy plants, little or no CO2 needed); ~20–40 medium (most plants, CO2 helps); over ~40 high light (demanding carpets and reds, CO2 and ferts basically required, and algae if you skimp). Deep tanks (over ~50 cm) need more at the surface to reach the substrate.
CO2 in ppm is about 3 × KH × 10^(7 − pH). It assumes your pH is set by carbonate hardness; buffers, peat or tannins shift the real figure, so confirm with a drop checker.
Around 15 to 30 ppm is the sweet spot. Above 40 ppm is risky and can stress or suffocate fish, especially at night; back it off if you see gasping.
A 4 dKH drop checker sits lime green at roughly 30 ppm, a live visual check. Use it alongside the calculation, since it lags real CO2 by an hour or so.
For how KH behaves in different source water, see water hardness, and for feeding those plants once your CO2 is dialled in, the fertiliser dosing calculator and beginner aquarium plants.
Estimates to help you plan, not lab measurements. The CO2 figure is the standard pH/KH relationship and is only as good as the assumption that carbonates set your pH. For real confidence, use a drop checker for CO2 and, if you are serious about high light, a PAR meter. See also water hardness for how KH behaves in hard water.