why not try a mammel, dwarf hamster, mouse, rat, or, what about some sort of bug, like a tarantula, or hmm a snake!
I think it is just about the right size for a small rodent or tarantula.
Snakes will always need something bigger as they grow up so not a good idea if you want it to be permanent.
I thought you were wanted something aquatic.
Turtles and tortoises will also grow too big. It could possibly fit small tree frogs or perhaps a lepoard gecko although I am not positive.