Depends on where you live and species of fishes. But I know quite a few people who make thousands off of rams and various species of apistogramma. Btw kribs breed like rabbits which is why they generally don't sell for much. However (and much to my amazements) bn pleco also breed like rabbits but still get a decent return (more specfiically some of the colour morphs like lemon blue-eye, super red, starlight as well as agreen dragon). I don't sell fishes but i can't keep my lemon's from breeding. My nannacara helps keep them mostly under control but the others i just give away.
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There are also cories which are relative easy to breed but have a high return like sterbai, orange laser, eques (listed in increasing value). Still if someone is going to breed fishes they should be responsible enough to mix up the genetics over time. In breeding is a real problem.