mg/L
CO2
What is it
Carbon dioxide (CO2) is the carbon source aquatic plants use for photosynthesis. Low-tech tanks rely on natural CO2, while high-energy planted tanks often inject it deliberately.
Why it matters
Stable CO2 lets plants use light and nutrients efficiently. Unstable or excessive CO2 stresses fish and shrimp, while too little CO2 under strong light often leads to algae and poor growth.
Interactions with other parameters
CO2 lowers pH while dissolved and is balanced by KH, light, plant mass and circulation. Oxygen can still be high in a well-run CO2 tank, but surface exchange must be managed.
Ideal ranges
| Tank type | Min | Max | Unit |
|---|---|---|---|
| Tropical community | 5 | 15 | mg/L |
| Planted high-tech | 25 | 35 | mg/L |
| Planted low-tech | 2 | 5 | mg/L |
| Shrimp tank | 5 | 15 | mg/L |
Out of range: what happens
Low CO2 causes slow growth, deformed tips and algae on older leaves. Excess CO2 causes fish gasping, shrimp climbing, lethargy and possible suffocation, especially when oxygen is also low.
Common Myths
- •More CO2 is always better; stability and animal safety set the limit.
- •A green drop checker guarantees perfect CO2 everywhere; dead spots can still be deficient.
How to measure
Use a drop checker as a rough visual guide and compare pH before and during injection. Watch livestock behavior because it is the most important safety indicator.
How to adjust
Increase CO2 slowly through bubble rate, reactor efficiency or flow distribution. Reduce it immediately if animals show stress and add surface movement or aeration.
Pro Tips
Start injection one to two hours before lights turn on so plants have CO2 at the start of the photoperiod.
Place the diffuser or reactor output where circulation carries CO2 through the entire tank.
