As already mentioned, platies, guppies, mollies and swordtails are up there with the rabbits. In a 5-foot, swordtails would add a lot of action and look good.
If you want something for the bottom, bristlenoses are also easy to breed, and those you can probably sell. Since they protect their eggs until after they are free-swimming, they don't need to be moved to a separate breeding tank, as long as you have caves for the parents to spawn in. My bristle male is currently fanning his fourth batch of eggs since this summer. I'm going to have to sell off at least all the males as they grow up- but that will finance further fish purchases.