Well, I would doubt the guys in the fish store are toxic. If you throw in wild, local ones, check with a good guide book or a naturalist society about them (I wouldn't say for your aquarium, collecting without a permit can be illegal). Off the bat, if it has big cheeks, those contain poison sacs. This is like the diference in head shape between a boa or a viper. A boa has a narrow head, a viper has a triangular head, showing the poison glands.