From The Aquarium Wiki
Hypoxia is oxygen starvation and is often fatal.
The oxygen requirements for different species varies widely. Generally torpedo shaped fish or rock clinging fish swim in fast moving water and so are used to high levels of oxygen.
- Fish swimming close to the water surface
- Breathing rapidly and not moving much
- Too high a bioload in the tank
- Too high a temperature
- High levels of bacteria (bloom) caused by organic pollution
- Chemicals like water conditioners added
- High levels of nitrite or ammonia causing gill damage
- Increase water surface movement (aerate the water)
- Lower water temperature if possible. See DO
- Reduce bioload in the tank
- Perform a water change
- Add small amounts of Hydrogen peroxide