Molly fish stay at the top of the tank due to sickness or poor water conditions. A damaged swim bladder is one of the most common reasons molly fish stay at the top of the tank.
Aside from floating at the water’s surface, fish with a damaged swim bladder will also struggle and swim erratically. Other illnesses that may be causing this behavior include parasite or bacterial infection, constipation, and bloating.
Alternatively, this behavior can also be caused by stress and sub-optimal water conditions, such as:
- Water that’s too cold or too hot
- Oxygen deficiency
- Excess waste in the water
- High concentrations of ammonia, nitrates, and nitrites