My common goldfish and koi devour oranges. I cut the orange into quarters and put two quarters in. Usually only takes em about five minutes to tear it up. Then I vacuum the bottom to collect uneaten pieces. It's cute because they leave the skin of the orange and also the transparent "skin" that is between the actual orange slices.
The oranges should only leech acid into the water if left in there for a long long time, and even then it's a fairly isignificant amount cause likely by the time it starts to leech, the fruit itself will probably be eaten.
Kiwi can also be fed to them and is also high in Vit. C.