In a low-tech planted tank, plant growth is usually limited by carbon. Light and nutrients are available; dissolved CO₂ is not, because it exists in water at only a few ppm. Injecting CO₂ to around 30 ppm removes that limit, and the change in growth is dramatic. So is the risk if the rate is set wrong.
What CO₂ actually does
Photosynthesis consumes carbon dioxide and water to produce sugar and oxygen. In an aquarium, the water is not the constraint, but the dissolved CO₂ is: natural equilibrium with the atmosphere gives only 2–5 ppm, and a busy planted tank exhausts that within minutes of the lights coming on.
Raise it to 30 ppm and plants can use the light and nutrients already present. Growth accelerates, internodes shorten, colours deepen and — crucially — plants out-compete algae for nutrients. This last effect is why CO₂ injection is often described as an algae treatment. It is not. It is a growth treatment whose side effect is that algae lose the competition.
| Dissolved CO₂ | Drop checker | Effect |
|---|---|---|
| 2–5 ppm | Blue | Natural equilibrium. Low-tech tanks live here. |
| 15 ppm | Blue-green | Noticeable improvement in growth. |
| ~30 ppm | Green | Target for high-tech tanks. |
| 40+ ppm | Yellow | Livestock at risk. Reduce immediately. |
| 60+ ppm | Yellow | Lethal to fish and shrimp. |
The equipment chain
A CO₂ system has six parts and every one of them can be the weak link.
Cylinder
Pressurised CO₂, typically 2 kg for a 60 cm tank. Must be secured upright — a falling cylinder can shear the valve off.
Regulator
Steps 800–1000 psi down to 1–3 bar. A dual-gauge, two-stage unit with a solenoid and a precision needle valve is the difference between a system you set once and one you fight weekly.
Solenoid
An electrically operated valve that lets a timer switch the gas off at night. Plants do not consume carbon in the dark, so running it then wastes gas and drives pH down for nothing.
Bubble counter
Turns flow into a countable rate. Fill it with mineral oil rather than water so the level — and therefore the reading — does not drift.
Diffuser
Ceramic disc that creates microbubbles. In-tank units give the finest bubbles; in-line units fitted to a canister return dissolve gas out of sight with no mist.
Drop checker
The only instrument that measures dissolved CO₂ rather than injected CO₂. Placed at the far end of the tank from the diffuser.
Setting the rate without killing anything
The method is deliberately slow. Start at one bubble per second for a 60 cm tank, and increase by half a bubble per second every two days, reading the drop checker at mid-photoperiod each time. Stop when it reads green. Then leave it alone.
The schedule matters as much as the rate: CO₂ on one to two hours before the lights, off one to two hours before the lights go off. Turning it on with the lights means plants spend the first hours of the photoperiod carbon-limited; leaving it on after the lights wastes gas and lowers pH overnight.
Why the bubble rate is not the measurement
Bubble counters measure what you inject. Losses between the diffuser and the water — surface agitation, plant uptake, degassing at the filter outlet — vary enormously between tanks. Two identical setups with the same bubble rate can end up at 18 ppm and 35 ppm.
The drop checker measures the result. It is slow, taking one to two hours to equilibrate, and it is coarse, resolving to three bands rather than a number. It is still the honest figure, and it is the one to act on.
The pH and KH relationship
CO₂ dissolved in water forms carbonic acid, which lowers pH. How far it falls depends on KH, the carbonate buffer. In a tank with KH of 4 dKH, 30 ppm CO₂ produces a drop of about one pH unit. In a tank with KH of 1, the same CO₂ produces a much larger drop and a much less stable tank.
This is why aqua soil and CO₂ injection interact badly if KH is not monitored. Aqua soil strips KH; CO₂ then moves pH freely. Test KH monthly, and if it reaches zero, add potassium bicarbonate to restore a buffer before adjusting anything else.