Hyla Cinera is immune to firebelly toxin. If you have an arboreal tank with a water portion these two will be able to stay in their own territories. I have seen some evidence that Hyla versicolor may also be immune although not with enough certainty. While other species may "Seem" to work, one can never be certain about low levels of toxin accumulating and causing cumulative damage and shortening the life of that other specie.