I second the gel food idea. [I have a recap and a calico oranda, and that's what they eat.] They had the swimming problems with standard food, and what causes it, I don't know. If it's just that I never soaked their food long enough, or that the food is made of things that shouldn't go in fishy tummies...
Their food is from a recipe on this board, and of the 1 cup of veggies, I make sure half of that is peas. No floaty problems since then, unless I humor them and feed them planktons, which are one of their 2 favorite treats [the other is orange slices.]
--Mav