One of my two fantail goldfishes can't stop from floating belly-up. He seems to struggle to resist it, and can hardly swim around. The other goldfish in the tank is fine. They are in a 10-gallon unplanted aquarium. With a pretty nice filter. The pH is about 7.5, no traces of ammonia or nitrite present, and the temperature ranges from 65-75F. Has anybody seen this happen before, or have any idea what is happening? How about treatments? He's such a cute (and for now) little fish.