i would breed large sailfin molies. find a breed that is 6"+ full grow, the bigger they are the bigger the babies and the larger the number of fry in each spawn. they can be cold water but unlike convicts they dont take care of their young so removing the fry would be important.
Try fathead minnows. They breed like crazy and are actually cool to watch. The male (hence the name) gets a fat head, and attracts females to his "den" to lay eggs and he guard them. They breed like cons, but grow faster, and can sometimes even have a greater success rate.