guppies, mollies, platies, or swordtails would work. Make sure you get 2 females for every male. You may even be able to buy a pregnant one from your fish store. Babies are great fun. Make sure you separate them from mom and dad right as their born, though, otherwise they will get eaten. The plastic breeder tank that floats in your tank with the "V" insert is good to put mom in before she give birth, so the babies can float down where mom can't get at them.