most fry that are easy to breed wont "fetch a fair price" because the market is usually filled with them. Supply and demand, to make money you have find a fish with high demand and low supply (which usually means not easy to breed). Mot of those that I can think of wont breed in a 20 gallon.
If you are up for a challenge look into something small and unique that might not be the easiest to breed. Those little orange mexican lobsters are fetching a good price in my area maybe look into something unique like that. They may be a challenge.