A pr of Apistogramma cacatuoides. They are a dwarf cichlid from South America and grow to a couple of inches. They are very peaceful and easy to keep and breed. You can sell the young to local fish shops or exchange them for other fish, plants or fish accessories.
If you're looking to breed something that is a complete change of pace from the fish in your signature, I'd suggest Bristlenose plecos. They are interesting fish and they seem to spawn on their own when they are ready to. The eggs and fry are large start out eating vegetables within the first few days. I enjoy mine a lot.