Depending on what you decide for the substrate. I went with bioactive and plants directly in it, so it's fairly deep 4-6 inches but you don't have to have as much. Just remember that your UVB lamp needs to be about 12 inches away from the top of the torts' shell in case you want to sit your lights on the top cover like I doThank you guys! Pearly you made my day.haha so how much dirt do I need to add? How many inches? Walls are 18" high