Had similiar fat belly syndrome with a dalmation molly and a swordtail. Moved them into a different tank, slowly introduced some salt, and got FRY! See if the other tank mates can tolerate some salt maybe?
On a funny note, I put the fry in the breeding net to especially get them away from this one inch albino frog but he managed to swim up the 16 or 17" of water and get over the 1" of net, and had a small snack of about 12 fry. I could have killed the stupid little thing if I had found him. MAYBE he'll go back to the LFS. Argh.