What I wanted to accomplish was a natural setting for my animal, like a little chunk of an African rainforest in my 4' diamter 2' deep horse trough. I was going to use a mix of 2/3 clean soil and 1/3 equal mix of sand and peat sphagnum and maybe 2" or so of gravel on the bottom for better drainage. Then many different kinds of live plants would be maintained inside the environment. Also, I heard of throwing a dozen or so worms in the soil to help reduce waste organisms and clean the soil, does that help?