Do you have a test kit? You should test for ammonia and nitrite. If you don't have a test kit, take a water sample to your local fish store and have them test it for you. Get the results in actual numbers... not 'fine' or 'bad', then post the results here.
Is there anything else you can tell us-- how long has the tank been set up, tankmates, any other symptoms? Is the fish behaving any differently than usual? Is your tank cycled?