fish equalliberium

Question
Why would an angelfish loose his balance?  When swimming it seems to do fine but when it stops it may turn over on its side, or upside down. Lately it has been laying upside down or on its side on the bottom, but when disturbed it swims away only to return to the bottom upside again.  this has been going on for a a couple of weeks,  Other fish seem to be ok.

Answer
Dear Ray,
Thanks for your letter. It sounds like it could be water temperature problems or overfeeding. When you clean the tank, you probably syphon out 10 - 20% of the water, then refill it with fresh bottled water. The temperature of the new water must match the temperature of the existing water or it will make the fish swim sideways. Temperature changes are very stressful to fish. It's best if you can get the new water to match the exact temperature before you add it to the tank. This is really tricky and time consuming, but necessary.
Overfeeding is another common problem. Fish don't need to eat much, but our tendency is to feed them too much. They need an ecosystem that includes friendly bacteria to break down fish waste and leftover food. I recommend using "cycle" in the tank, which is essentially friendly bacteria, i.e. "organic biology," that you can buy at the aquarium store. Add it to the tank according to instructions on the label, then keep it in the fridge for all future water changes.
I suggest that you stop all feeding for 24 hours, add the Cycle, then drastically reduce the amount you feed them on a daily basis. Don't worry - even a couple flakes a day per fish is plenty until the fish get really big.
My page for easy aquarium care is posted here:
http://steamboats.com/aquarium.html
If you have any questions after reading this, please write back.
If you are careful about temperature, if you use Cycle during water changes, and if you reduce the food, you should see an improvement. If not, write back again.
Sincerely,
Nori