CO2 Injection in Planted Tanks

Why injected CO2 boosts plant growth, how a drop checker reads the ~30 ppm target, the pH-KH-CO2 relationship, and the safety risks of overdosing.

Why CO2 is added

Carbon is the most important element for plant growth, and CO2 is the carbon source plants use in photosynthesis, the process that converts water, light, and carbon dioxide into carbohydrates and oxygen. CO2 is described as the most important nutrient for aquatic plants; without it they cannot photosynthesize. Adding extra CO2 can dramatically improve plant growth.

The target level

A CO2 concentration of about 30 parts per million (mg/L) is widely used as a safe and effective target for most fish and invertebrates. By plant difficulty, medium plants benefit from roughly 10 to 15 mg/L of CO2 and more demanding plants from about 15 to 30 mg/L.

Using a drop checker

A drop checker holds an indicator solution with a known 4 dKH reference. CO2 diffuses from the tank water into this solution and changes its colour: it turns green when the dissolved CO2 in the solution reaches roughly 30 ppm. The reading lags real-time tank levels by an hour or more, so it shows a trend rather than an instant value.

The pH-KH-CO2 relationship

In water with a stable carbonate hardness, dissolved CO2 can be estimated from pH and KH. A solution at 4 dKH adjusted to 30 ppm of CO2 settles at a pH of about 6.6, which is the basis for both the drop checker reference and pH-KH-CO2 reference tables. Carbonate hardness must be stable for this estimate to hold.

Safety and overdosing

CO2 is highly toxic when overdosed and is among the biggest causes of livestock loss in planted tanks. Excess CO2 causes fish to gasp, dash, and jump, and can be fatal. The toxicity threshold depends on dissolved oxygen: in low-oxygen water livestock are more sensitive, while higher oxygen raises the safe limit.

Night-time management

The greatest risk arises during the dark period, when plants respire, produce CO2, and consume oxygen. To avoid intoxication, the CO2 supply should be set to switch off shortly before the lights go out and resume before they come on.