With reduced filtration and frequent changes, and a heater set to the max room temp, you actually have quite a few options. There are dwarf frogs. You could probably do one male guppy without trouble. I would go to a nearby pond and throw a scoop of mud in it. It's like live rock. I got fly larvae, snails of various types, various nematodes, fairy shrimp and you can seed it with tubifex. Feed with algae wafers and bits of meat. I have one of these going at about eight months old with good results. Now it's even turned green with infusoria! So rockin'! I also do very infrequent water changes on it to retain stability. Everything in there thrives. It's great. I even raised a tadpole in mine, though he totally decimated the daphnia population. Now the top of the food chain are the fairy shrimp. They are sooo frickin' cool, and almost zero maintenance.