Get a small pair of Honduran Red Points. I used to breed them in a 20 high. I would routinely sell the fry for $20-25 for ten 1" fish on aquabid. The price included USPS Priority Shipping, which cost me about $10 per box.....I made around a buck per fish when you factored in bags, bag buddies, gas, etc. It was fun, and I would estimate I made about $500 from my pair of HRPs, if you include Craigslist sales!!!!
Try to get a small, established pair, though. A 20 is kinda small, and if they are strangers, good chance of a casualty.