I have goldfish, koi, and a few snails (the large ones from PetSmart... fish seem to have eaten all the small ones). I bought several of those electric ice-blue, spotted mollies, but they never lasted long - either birds, 'coons, or they don't like Houston summers.
Between all that, I have no problem with algae. I think the fish spend a good part of their day picking it off the sides of the pond liner.