I had a Wakin that did something similar - it spent a lot of time floating, mostly upside down, but when I got near it went to the bottom (with a lot of effort) and seemed to be eating and swimming around okay other than floating to the top occasionally. The vet said that it probably had a problem with it's float gland or whatever it's called and a possible problem internally. Eventually he had gotten some sores and didn't look too well, so I sent him to the pond in the sky for fear that the others might get whatever he had. The others didn't hang around him much, so they knew something was up. He was my favorite, too....he was orange and white , sort of striped, and his name was, if you hadn't guessed already, - Nemo.