Hi Snotirl
The thing about water and oxygen is easy. As water heats up its ability to disolve oxygen goes down, or in other terms the saturation level of oxygen in warm water is lower than in cold water. This means that fish have to work harder to get the correct level of oxygen in warm water than in cold water. On hot days pond fish tend to stay around water featurs (water falls, fountains) as there is more oxygen in these parts of the pond.
Try droping the temp of the tank, and see if that helps.
Another posibility is that they have swimblader or constipation problems. Do they stay there all the time, or just at certian times of the day?
Dolphin