You really cant keep anything long-term as far as inverts are concerned (i cant keep anything in my dwarf puffer tank), puffers are really only meant for FOWLR tanks. If you want to waste some money and time, which seems to be the case, you can buy peppermint shrimp or banded coral shrimp. Im warning you now, they will not survive long (in fact, id be suprized if they lived long enough to reach the bottom of the tank).