I have an 18"x 18"x 24" terrarium (Length, width, height) for a White's tree frog. How many bags of Hydro-balls would I need to cover 1-2" of drainage for it. The bottom of the terrarium is 18" x 18". I was thinking maybe like 2??

Zoo Med HydroBalls Expanded Clay Terrarium Substrate - Sand - Substrate & Bedding - PetSmart