While fish breeding won't replace your day time job, it could be a nice source of money. Depending on your experience in the hobby, what you breed varies.
If you're a beginner, livebearers or kribensis cichlids would be a good idea. While most livebearers wouldn't make too much money, if you got some rare color form such as a koi swordtail, you could make a lot. Kribensis are a bit harder, but they are more profitable. I have seen young juveniles sold for $8 each.
If you have spent a good amount of time in the hobby, your options are much wider. There are more cichlids you could breed, like angelfish. Also, there are many egglaying species. Tetras, danios, and rainbowfish are just a few of the fish that reproduce by scattering eggs. In their breeding tanks, you will want to provide bushy plants like java moss and cabomba. Raising the fry of all egg laying fish is harder because you must remove the parents and feed the fry small live foods like infusoria or brine shrimp.
On the other hand, you could try breeding invertebrates. Nerite snails are always desirable. If your water is extremely hard, you could try breeding them. Just beware that the young have a planktonic stage, so don't put in any fish. If your water is on the hard side, you could also try red cherry shrimp. They multiply very easily, and you could sell them $4 each at most. If your water is softer, you could try other species like crytal red shrimp. Be aware though, that some shrimp warieties interbreed. If they are in the same genus, this will happen. For example, if you put crystal red shrimp, in the genus Caridina, with red cherry shrimp, which are in the Neocaridina genus, they won't interbreed. However, if you put the red cherry shrimp with snowball shrimp, also in the genus Neocaridina,they would interbreed. All the inverts mentioned do well in planted tanks, and without heavy metals, especially copper in the water, because these are deadly to them.
In the end, if you want to breed fish, you will need to make sure you have two tanks for each species of fish, one for the breeding one for the rearing.