I never thought i would do but i did, so my vote would be a scorpion. They require very little. Just get a heating pad and use ur tank light with some sand or other substrate and ur fine. In a 20 u could 2 scorpions. I buying my second one tomar for my 10 gallon and when bigger i will put them in a 20 gallon long. My be gross to you like it was for my fiance but she loves it now
Well I decided that once my current grow outs are big enough to go into the other tanks that this tank will be a planted tetra tank. I'm going to put like 10 different kinds of tetras and I may use it to grow some discus for a little while as well. Still have 2 or 3 months before the grow outs are moved though.