Dissolved oxygen
Dissolved oxygen is rarely tested in home aquariums and is behind a large share of unexplained losses. Solubility falls as temperature rises — water at 30 °C holds roughly a quarter less oxygen than at 20 °C — and biological demand rises at the same time.
In one sentence
Oxygen enters the water at the surface and is consumed by fish, plants at night, and above all by the bacteria in the filter and substrate. Warm water holds less of it, which is why heat and oxygen problems arrive together.
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 |
|---|---|---|---|
| Well oxygenated | ≥ 7 | target | Comfortable for freshwater fish. Note that this is close to saturation at tropical temperatures — warm water cannot hold much more, so a higher figure is not an achievable target. |
| Adequate | 5–7 | acceptable | Workable for most community species, but with little margin. Active species and heavily stocked tanks feel the difference. |
| Marginal | 4–5 | watch | Increase surface agitation. Fish may show no signs yet, and this is the band in which a hot afternoon or a stopped filter becomes an emergency. |
| Low | 3–4 | act | Expect gasping at the surface, largest and most active fish first. Increase aeration immediately and cool the tank if it is above the species' range. |
| Critical | ≤ 3 | urgent | Deaths are likely and are usually attributed to something else. Maximum surface agitation now. |
Where the oxygen actually goes
Fish are usually the smallest consumer. The nitrifying bacteria in a mature filter consume a substantial share, and decomposition in the substrate consumes more. A heavily stocked tank with a large mature filter can be close to its limit on a hot day without anything visibly wrong.
Increasing it
- Surface agitation is what matters, not bubbles. An air stone works because the rising bubbles disturb the surface, not because oxygen diffuses from the bubbles themselves.
- Lower the temperature. This is the largest single lever in a heatwave.
- Reduce the bioload temporarily by not feeding.
- Check for a bacterial bloom — cloudy water is itself a large oxygen consumer.
How it is measured
- Dissolved oxygen meter
- Liquid DO test kit (uncommon in the hobby)
Typical aquarium range
- Typical
- 6–9 mg/L
Common misconceptions
| Often said | What is actually the case |
|---|---|
| Plants oxygenate the tank at night. | Plants respire at night and consume oxygen. In a heavily planted tank the lowest oxygen of the day is just before lights-on. |