I recently read something -- pertaining to leopard geckos -- that suggested that impaction by sand was usually caused/fostered by dehydration in the animal that made it harder to pass the sand through the system. I'm actually wondering if/to what extent that might be true of amphibians. (I wouldn't use sand for most amphibians, though, as their skin is so sensitive, and sand is so coarse and dry. I was wondering for the desert-type species that John referenced earlier in this thread. I've never kept low-humidity toads, so I have no personal experience with sand substrates and toads.)