A super easy one to breed is the Jewel Cichlid. They aren't worth a TON, but I used to breed them and sell them to a local fish store. Made a lot more than I put into it! But you have to make sure you have a reliable customer, cuz there isn't a huge demand for them in the real world, best to find a store that will buy them. But you can buy the initial school for 2-5 bucks a piece and they breed very easily. And the males get BEAUTIFUL during breeding time!