HydroBalls Lightweight Expanded Clay Terrarium Substrate - 2.5 lbZoo Med HydroBalls Lightweight Expanded Clay Terrarium Substrate can be used as a base layer underneath Zoo Med's Eco Earth coconut fiber substrate or other substrates to create an underground water table in naturalistic terrariums. Add water to create a natural "aquifer" and increase humidity levels for all tropical species. Excellent substrate for live plant root growth in naturalistic terrariums. Use for filtration for a waterfall.