CO₂ for Planted Tanks: Setup and Safe Levels

Adding CO₂ transforms plant growth, but it gives you a number to watch carefully — too much and the fish suffer. Here is how to run it safely.

Plants use CO₂ to grow, and in a bright planted tank the natural supply runs short. Adding CO₂ lets them grow faster and fuller — at the cost of one number you must keep in a safe range.

Why add CO₂?

With CO₂ and enough light, plants out-compete algae, grow denser and colour better. Without it, strong light just feeds algae. CO₂ is the missing piece in a high-light planted tank — but it is optional for easy, low-light plants.

How it works

Pressurised system — a cylinder, regulator and diffuser; the most controllable.
DIY yeast — cheap, but inconsistent; suits very small tanks.
A drop checker shows the CO₂ level by colour and is the simplest safety aid.

Keeping it safe

Aim for 20–30 ppm — use the CO₂ calculator and a drop checker.
Turn CO₂ on before lights and off at lights-out.
Watch the fish: gasping at the surface means too much CO₂ — increase surface movement and reduce it.
Keep it stable, rather than chasing a perfect number.

Do you need it?

If you keep easy plants under basic light, no. CO₂ pays off only when light and plant demands are high enough to use it — see lighting for plants.

Pressurised vs DIY CO₂

There are two ways to add CO₂ to a planted tank. A pressurised system — a gas cylinder, a regulator and a diffuser — is the controllable option: once set, it delivers a steady, measurable flow and lasts months before refilling. It costs more up front but is the reliable choice for anything but the smallest tank. A DIY yeast system is cheap and can work in a nano tank, but the output rises and falls as the yeast culture ages, so the level is never steady.

Whichever you use, the gas is dissolved into the water by a diffuser or an inline reactor, and it is measured in parts per million (ppm). The goal is a stable target, not the highest number you can push.

Setting the level

The usual target is around 20–30 ppm. The simplest way to check it is a drop checker: a small glass device filled with a pH indicator solution that turns green in the target range, blue when the CO₂ is low and yellow when it is too high. Treat a yellow reading as a warning, not a goal. A pH and KH reading can be used to estimate CO₂ with the standard formula, but the drop checker is more forgiving for everyday use.

Change the level slowly. Turn the bubble rate up a little, wait a day, and check both the drop checker and the fish before adjusting again.

Timing and stability

Plants can only use CO₂ during the hours they are lit, and fish do not need it at night. Run CO₂ on about an hour before the lights come on and off at lights-out, using a solenoid valve on a timer. Leaving it running in the dark raises the level with no benefit and stresses the fish. Stability matters more than any single number: a steady 25 ppm suits plants far better than a level swinging between 5 and 40.

Reading the plants and fish

Plants tell you when the balance is right: steady, compact growth, good colour and less algae. Fish tell you when it is wrong: gasping at the surface or hanging near the top means too much CO₂ — increase surface movement and reduce the injection immediately. If plants grow slowly and algae dominates despite strong light, CO₂ is likely the limiting factor. Always watch behaviour as well as the reading, because the fish react before a checker turns yellow.

Never let it run at night

Plants release CO2 in the dark instead of taking it in, so leaving injection on overnight pushes the level up with no benefit and can harm fish by morning. A solenoid valve on a timer, switching CO2 off at lights-out and on an hour before lights-on, is the standard safeguard. If you run a DIY system without a solenoid, keep surface movement high so gas exchange helps buffer the level overnight.

What CO₂ level is safe?

Roughly 15–30 ppm suits plants; above about 30–40 ppm risks the fish. Always watch behaviour, not just the reading.

Can CO₂ kill fish?

Yes, if overdosed — fish gasp or die. Use a drop checker and increase surface movement if in doubt.

Is it worth it for a low-light tank?

Usually not. Easy plants grow fine without it.

Tools for this guide

CO₂ calculator

Sources and research notes

Guidance follows standard planted-tank practice; the CO₂–pH–KH relationship and a drop checker are the usual tools for keeping levels safe.

Written and edited by the Tankwise Editorial. Every guide answers one real fishkeeping question, is checked against the sources listed above, and is updated when guidance changes. We are an independent editorial desk, not veterinary or aquarium professionals — read how we research and update our guides.