Without a doubt convicts. If you have a male and female within a month you will have hundreds. And be careful not to put them in a tank with other fish. They will kill everything to breed. Males get a pretty cool hump on their head with some decent finnage and females look great when spawning. Other than that I would say blood parrots. They tend to breed fairly easy too. Just make sure not to buy "dyed" or "tatooed" parrots since that is cruel.