Hi Jakezori,
When they were fairly new to the hobby, and not seen often in aquarium shops, I bought a pair of Madagascar Rinbowfish, and after 6 months they began to spawn a lot. I would harvest about 50-75 eggs every 3 months or so and raise them up with newly hatched brine shrimp until thay were about 1.5-2 inches and showing good color. I found that several of the area aquariums were willing to give me $5 each for the young fish. That netted me about $250 every 3-4 months, which paid for my hobby for several years.
What you need to do is select a fish that is attractive, somewhat hard to find perhaps, that you can sell to the stores for half of what they sell them for and still have it be a profitable venture to you after you subtract your costs of raising and feeding the fish until they are ready to be sold. I hope this helps. Good luck!
Rainbow Kurt