I was doing some research for my 2.5G planted (soon-to-be), and I found some nice information.
I was looking on different forums, and see what they said from experience/wholesalers, etc. You *might* be able to get a very small shoal of dwarf rasboras, such as Boraras micros, Boraras merah (best choice IMO; they stay around .8"), Boraras brigittae, or Boraras maculatus. Maybe 1 Dwarf Puffer, as long you do weekly water changes of 50% or more, water vacs each week, and feed live snails/blackworms or bloodworms/frozen wood. You need live plants and plenty of things he can explore in, or else you shouldn't have one in a 2.5. I also read that a single, or pair of Dario Dario may work. You can get overrun by babies if they are happy though.[/i]