What CO2 ppm should a planted tank target?

The widely used target for healthy plant growth is around 30 ppm of dissolved CO2. High-light, densely planted tanks push toward that figure, while low-tech or lightly planted tanks do well at 15 to 20 ppm. Above 30 ppm risks gassing fish, so 30 ppm is the practical ceiling.

How is dissolved CO2 ppm related to KH and pH?

Dissolved CO2 follows the relationship CO2 ppm = 3 times KH times 10 to the power of (7 minus pH), where KH is in degrees. This calculator uses it to convert your measured KH and pH into the CO2 your water currently holds, which you compare against the target.

How is daily CO2 consumption estimated?

It scales the target ppm by tank volume to get the dissolved CO2 mass at equilibrium, then multiplies by a turnover factor for plant uptake, degassing at the surface, and photoperiod. It is an estimate to size a reactor and cylinder, not a precise gas-flow measurement.

Why does light level matter for CO2?

Light drives photosynthesis, and photosynthesis consumes CO2. Higher light increases plant demand, so high-light tanks need a higher injection rate and a reactor with enough contact time to dissolve gas before it escapes. The calculator raises the demand factor with light.

Is bubbles per second an exact measurement?

No. Bubble size varies between diffusers and check valves, so bubbles per second is only a relative starting point. Always confirm actual dissolved CO2 with a drop checker or KH and pH measurement and adjust the needle valve from there.

Planted Tank CO2 Reactor Sizing Calculator

Sizing CO2 for a planted tank means hitting a target dissolved concentration without gassing your fish. This calculator recommends a target ppm for your light and planting level, converts your current water chemistry to its existing CO2, and estimates daily consumption so you can size a reactor and cylinder.

How it works

Two relationships drive the tool. The first converts water chemistry to dissolved CO2; the second estimates demand:

CO2 ppm (current) = 3 × KH × 10^(7 − pH)
target ppm        = base(light, density), capped near 30 ppm
dissolved mass    = target ppm × volume_litres / 1000   (grams at equilibrium)
daily consumption = dissolved mass × turnover(light) × photoperiod_factor

Because CO2 continuously degasses at the surface and is consumed by plants under light, the reactor must inject more than the equilibrium mass — that is the turnover factor. The bubble-rate suggestion scales with that daily figure.

Example and tips

A 120 litre, medium-density, high-light tank targets roughly 30 ppm. The dissolved mass at equilibrium is about 3.6 g, and with a high-light turnover the daily consumption estimate lands in the low tens of grams, suggesting a reactor rated for that turnover and a needle valve set to a few bubbles per second to start. Always confirm real dissolved CO2 with a drop checker, ramp injection slowly, and turn CO2 off at night when plants stop consuming it to avoid overnight build-up.