With many common fish, the difficulty is not to get them to mate, but to find place for all the fry! You need enough grow-out space for the fry to grow to a sellable age- that's lots of fresh water! And while the fry are little, you need enough space to either protect the fry from the parents or your other fish from the parents, depending on whether you have the kind of fish who look after their young or not. Fish that look after their young (like cichlids) tend to get extremely aggressive towards other fish at these times. And then you need someone who will take them off your hands. It is not like most fish will have one or two babies- one batch may well go into hundreds.
Convicts are some of the easiest cichlids to breed, but this also means that the shops may already be overrun with convict fry. Goldfish are also quite easy, but they need quite a bit of space.