Substrate

Substrate is the material on the floor of the enclosure — for most terrestrial amphibians a soft, moist, non-swallowable bedding like coco fibre or topsoil (never gravel).

The right substrate lets a burrowing amphibian dig, holds humidity, and — crucially — won't cause impaction if a bit is swallowed with food. Gravel and other hard, loose materials are a leading avoidable killer.