I've heard that white mountain minnows do well in biOrbs, or you could buy a heater and keep a shoal of zebra danios, you could set it up as a betta tank, and have a single male and maybe a couple of apple snails or ghost shrimp, you could also keep a small shoal of some kind of tetra in there. I can't think of much else you could do with one. I have a baby biOrb (15 litres) which I was given for free, I'm planning to set mine up as a betta tank, with just a betta and like two plants after its cycled =p