Making a profit with a single species is not easy to do. Some of it's a moving target that follows the latest hot fish. What's hot now could lose interest in 6 months or a year. Some fish that are less common, like festae or other larger, more aggressive fish, also have a limited market, which is part of the reason they're less common-- you might do ok with some of these if there's local interest, but before long you exhaust most of that market. So rarity doesn't guarantee a market opportunity. You're not going to move the hundreds or thousands of fry some large, aggressive new world cichlids can produce.
There's always selling online, which has it's own set of challenges. Local or regional, like in a cichlid association, and it all depends on what you have, again some fish will have a limited market. Going to aquabid or ebay and now you're competition includes experienced sellers with an established reputation, you might find a niche there if you stick with it and have the right species-- or not so much.
The latest hot new fish can be profitable for a while, until everyone else and their dog is also breeding and selling them and the price goes down. Some fish do have staying power and can make some money for a successful hobby breeder, angels and discus already mentioned, Cyphotilapia, especially Zaire gibberossa (but these are harem breeders), also some African cichlids that have a niche but it's a steady niche (tropheus, for example).
Beyond that you can pick a fish you like that has staying power and more than a small niche market. You won't make a killing but probably always be able to sell some, nice severum species, oscars, some eartheater types, for example.