I initially used two types of sediment, with the finer stuff on top. I noticed water was not penetrating more than an inch down, and the entire layer of thicker substrate was untouched. Also, the snails were unable to move in the fine stuff. I tried to stir it as gently as possible in an effort to bring some of the coarser sand to the top, but even after just doing a small area, the water clouded pretty badly. That unfortunately led to my amphipods and smaller porcelain crabs being unable to breathe apparently, for they all died.

For reference, the substrate on top that is extremely powdery and apparently too fine for even small snails to locomote upon is CaribSea Arag-Alive 20-Pound Special Grade Reef Sand, Bahamas Oolite. I probably should have used only the CaribSea Arag-Alive 20-Pound Special Grade Reef Sand, Bimini Pink.

Tank clouded by oolite fine substrate, leading to death of small crabs and amphipods

The porcelain crabs and a bunch of amphipods behaved erratically and ended up in a pile in this valley before they died.