How much oxygen can water hold?
| Temperature | Fresh water | Seawater (35 ppt) |
|---|---|---|
| 15 °C (59 °F) | 10.1 mg/L | 8.1 mg/L |
| 20 °C (68 °F) | 9.1 mg/L | 7.4 mg/L |
| 24 °C (75 °F) | 8.4 mg/L | 6.9 mg/L |
| 25 °C (77 °F) | 8.3 mg/L | 6.8 mg/L |
| 26 °C (79 °F) | 8.1 mg/L | 6.7 mg/L |
| 28 °C (82 °F) | 7.8 mg/L | 6.4 mg/L |
| 30 °C (86 °F) | 7.6 mg/L | 6.2 mg/L |
How the risk score works
| Factor | Points |
|---|---|
| Water 27–29 °C / 30 °C or warmer | +1 / +2 |
| Saltwater | +1 |
| Moderate / heavy / overstocked | +1 / +2 / +3 |
| Gentle ripple / still surface (an air stone takes off 2) | +1 / +3 |
| Tight lid without strong surface movement | +1 |
| CO₂ injected at night | +2 |
| Medicating the tank | +1 |
Worked example
A heavily stocked freshwater tank at 28 °C with a gentle ripple and a tight lid scores 1 (warm) + 2 (heavy stocking) + 1 (gentle surface) + 1 (lid) = 5, moderate risk. Water at 28 °C holds at most 7.8 mg/L.
If CO₂ injection also runs overnight, the score rises to 7, high risk: plants stop producing oxygen in the dark while fish, plants and bacteria keep using it. Merck notes that fish killed by low oxygen are often found in the early morning, when oxygen is lowest.
How to raise oxygen in a fish tank
- Break the surface. Point the filter outlet upwards or add a spray bar or air stone.
- Lower the temperature to the bottom of your fishes’ range during heatwaves.
- Turn CO₂ off at night with a solenoid timer.
- Reduce stocking and feeding if the tank is overcrowded.
- Clean the filter gently so flow stays strong.
- Keep a battery air pump for power cuts.
