Most invertebrates need algae to feed on and more room to roam. but if there is algae in your tank then you could keep small snails such as a turbo snail. Buy if there isnt then it might be possible to keep a cleaner shrimp, and occasionally feed it. If you really wanted you could always place store bought algae sheets in the tank, allowing you to keep all sorts of critters. but with no filtration i would definatly not try it unless you start filtering it.