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.

What is the Planted Tank CO2 Reactor Sizing Calculator?

Enter tank volume, plant density, and light level to compute target CO2 ppm, a bubbles-per-second injection rate, and an estimate of daily CO2 consumption to help size a pressurized reactor. For advanced planted tank hobbyists. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Planted Tank CO2 Reactor Sizing Calculator

Name: Planted Tank CO2 Reactor Sizing Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

Understanding the KH–pH–CO2 triangle

The formula CO2 = 3 × KH × 10^(7 − pH) is a cornerstone of planted tank chemistry. It tells you the dissolved CO2 concentration your water currently holds, derived from two parameters you can measure with inexpensive test kits.

For example, at KH 4 dKH and pH 7.0 the calculation gives 3 × 4 × 10^0 = 12 ppm CO2 — a low level. Lower the pH to 6.8 at the same KH and dissolved CO2 jumps to roughly 19 ppm. This is why experienced planted tank keepers watch pH drift during the photoperiod: rising pH during the day reflects CO2 drawdown by plants, while a stable or climbing pH after lights-off signals CO2 is not accumulating.

A drop checker uses a similar principle — the reagent inside equilibrates with the tank water and changes colour as CO2 shifts. Green typically indicates roughly 30 ppm; yellow is too high, blue too low.

Worked example

A 120-litre tank with medium plant density and high lighting might target about 30 ppm dissolved CO2 (the practical safe ceiling for most fish). The dissolved CO2 mass at equilibrium is approximately 3.6 grams. With high-light turnover over an 8-hour photoperiod, daily consumption could reach 15 to 25 grams depending on plant mass and surface agitation. That consumption figure helps you estimate cylinder life: a standard 2 kg CO2 cylinder might last a few weeks to a couple of months depending on your setup.

Light level and why it matters

CO2 demand tracks light intensity closely because light drives photosynthesis — the biochemical reaction that consumes CO2 and produces the oxygen bubbles you see pearling off healthy leaves. A low-tech, low-light tank with slow-growing plants needs only 15–20 ppm and a slow injection rate. A high-light, densely planted Dutch or Nature Aquarium style tank needs near 30 ppm and a reactor sized for significant throughput.

Practical reactor and injection tips

Turn CO2 off 30–60 minutes before lights-out. Plants stop photosynthesising at the end of the photoperiod; CO2 keeps degassing into the tank after that, and overnight build-up can stress or kill fish by morning.
Start slow. Set the needle valve to a couple of bubbles per second and observe fish behaviour for 24–48 hours before increasing. Gasping at the surface or clamped fins are early signs of excess CO2.
Reactor vs. diffuser. An inline reactor dissolves CO2 under pressure before water returns to the tank — more efficient and no mist. A ceramic diffuser produces micro-bubbles that dissolve in the water column — simpler but less efficient at high flow rates.
Surface agitation and CO2 loss. Heavy surface ripple strips CO2 rapidly. Adjust spray bar angles and powerhead outputs so there is gentle surface movement for oxygen exchange without the choppy agitation that wastes your injection.