This sounds like a case of water toxicity.
The first and most important diagnostic tool in fishkeeping is water testing. A standard water test kit is sufficient for testing. The things that should be tested for are the presence of ammonia, nitrite and nitrates. Ammonia and nitrites should test at -0-. They are both lethal to fish. Nitrates should test at 40ppm or lower.
If the water tests fine it's on to the next step
but if it doesn't test out ok then it should be corrected via frequent water changes.
If the water tests out OK then the fish's symptoms would indicate a bacterial or parasitic infection which should be treated with medicine.
I recommend treating with a medicine called Maracyn. It's a good all around medication that will treat a variety of ailments. Just follow the directions on the product label for its use.
Below is a link to a web page showing what the product looks like. Good luck!
www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=4838&N=2004+113021