very personal one this. to me its a Crayfish, others like shrimp or snails. firebelly newts, but they are temperate, so are no good here.
if you want safe! apple snails and African Dwarf frogs are worth a look. safe for the fish, not necessarily for the invert, ghost/wood shrimp. crabs are a no no, not coz they eat fish, or visa versa, but because they are land critters, not suited to an aquarium.