you could put in an african clawed frog but its probably best to offer some land are if you do that. the land area could be a small floating log or something similar.
Alternatively, you could get some shrimps such as amano shrimp or ghost shrimp. I would steer you away from any kinds of freshwater crabs as they can be very aggressive, territorial and carnivorous as my friend unfortunately found out the hard way.
HTH!
reef_guy
