I wouldnt call fluorite black sand a loose substrate. I have a dwarf hairgrass carpet in mine, and it sends its runners out above the surface of the substrate, I assume because its too tight to grow into.
Its also over dirt, and quite thick, 1.5 inches of dirt, 3 inches of fluorite black sand, and I get no gas pockets, Ive tried poking around to find them. I have about 500 malaysian trumpet snails though.
Also just fyi while were were talking about carpets in diff types of substrates, I put a few clumps of the hairgrass in some fluval stratum in my ebi, and it spreads much better, and grows thicker and taller. I also put some clumps into a home depot sand over dirt tank, and it doesnt grow at all. That one might be due to low lighting though.