Carbon dioxide in the aquarium
In a low-light tank, the CO₂ produced by respiration is usually enough. Increase the light without increasing the carbon and the plants cannot use the extra light — but algae, which need far less, can. That mismatch is the mechanism behind most algae problems in planted tanks.
In one sentence
CO₂ is the carbon source for plant growth and is normally the limiting factor in a brightly lit planted tank. It also forms carbonic acid, so injecting it lowers pH.
How to read a measurement
Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.
| Band | Range | Verdict | What it means |
|---|---|---|---|
| Ambient, no injection | ≤ 5 | acceptable | What an uninjected tank sits at through gas exchange with the air. Fine for low-light and slow-growing plants; the limiting factor for demanding ones. |
| Injected planted-tank range | 15–30 | target | The band planted-tank practice aims for. Reached gradually over the photoperiod, with the injection off at night when plants respire rather than photosynthesise. |
| Elevated | 30–45 | watch | Fish begin to show stress: surface breathing in the afternoon, and reduced activity. Dissolved CO₂ impairs a fish's ability to unload its own respiratory CO₂ at the gill, so this looks like an oxygen problem and is not one. |
| Dangerous | ≥ 45 | urgent | Turn the injection off and increase surface agitation. Fish losses from CO₂ overdose typically happen in the late afternoon and are routinely misdiagnosed as disease. |
Estimating dissolved CO₂
The standard estimate is CO₂ (mg/L) ≈ 3 × KH (°dKH) × 10^(7 − pH). It assumes the carbonate system is the only significant buffer, and it over-reads badly where tannins, phosphate buffers or commercial pH adjusters are present. A blackwater tank at pH 5.5 is not carrying 90 mg/L of CO₂.
A drop checker containing a known 4 °dKH reference solution avoids the assumption by measuring the CO₂ that has equilibrated into a reference water rather than into the tank water. It lags by an hour or two, which is a feature rather than a fault.
Safety
- Excess CO₂ suffocates fish. Gasping at the surface shortly after lights-on is the classic sign.
- Turn injection off overnight; plants do not use it in the dark and it accumulates.
- Increase surface agitation at night, which drives CO₂ off and brings oxygen in.
How it is measured
- Drop checker with a 4 °dKH reference solution, which lags actual conditions by an hour or more and indicates rather than measures
- Calculation from pH and carbonate hardness, which is only valid where carbonate is the sole significant buffer — see the tool's stated assumptions
- Watching the fish, which is the least precise method and the one that matters most
Typical aquarium range
- Typical
- 2–30 mg/L