If you want to make money from it then a CRS (crystal red shrimp) tank would be profitable providing you can bring yourself to cull the undesirables or you could give them away for cheap as long as the don't fraternise with breeding stock. A betta sorority or a single male betta or you could put a tank divider in an have 2 male bettas, dwarf puffers seem popular lately as well. There is a thread here for more options.