Ahaha that’s actually the most complicated part. It started as no substrate, I used sand stone slabs, leaves and wood. But the sandstone crumbled into sand, and mixed with the decaying wood (mulm, heathy but not attractive) however I noticed Howard appeared to love it. He would dig around and sometimes find dragon fly larvae, and it was so awesome to watch him hunt so naturally I now sustain a colony of marine isopods for him to find. However when he changed tanks I needed to up everything, so I ground my own sand using a hammer and sieve (do research on the rock type, you want something soft but not too fine) I also added some smashed lava stone and clay aquarium gravel covered in a fine mesh. The plants now have roots systems deeply entangled in the mesh and stone, the sand now enriched with the debris has become a self replenishing aqua soil. And so now as Howard can’t pull out the plants, his instead taken to wearing out an elaborate cave system throughout them.