We did a large enclosure for Dwarf Crocodiles kind of like that. This enclosure has a 3/4 osb floor with visquene over that, and the we built it up with dirt woodchips etc.. The water area was a kiddie type pool.

It can out pretty good but we recently removed the dirt woodchips and some large tree limbs ... we started having a bug problem within the enclosure and then even started growing these huge wild mushrooms. LOL Crazy! Bugs not so cool expecially since its a indoor enclosure. Not sure if the bugs originated from the bagged dirt we used or maybe from the tree limbs.

I probably wouldnt use dirt or tree limbs again unless I cook it all in the over first to make sure there is no bugs or anything like that.
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Great Lakes Zoological