Swim Bladder Disease
Signs of a problem: Fish cannot control their position in the water, sometimes only manage to rise from the bottom with maximum effort, sometimes cannot leave the surface, and may even float upside down at the surface.
Affected fish: If the problem is not caused by disease, then the issue may be due to malfunction of the swim bladder. Such problems are usually limited to deep-bodied fish, especially cyprinids. Scale-less fish seem to be especially susceptible.
Recommended treatment: There is no effective treatment for this condition. Because this disease is brought on by cold water, slightly raising the temperature along with a moderate salt solution can often relieve the symptoms. Feeding daphnia or worms to improve digestion can help with treatment.