Hello,
Here is the scoop on sand sifting gobies. They feed constantly on the fauna (little live things in the sand) they scoop it in their mouths and expel the "sifted" sand from what looks like it's gills...Golden sleeper gobies swim as they do that, and get sand everywhere on the rock and corals. (I loved mine). There are others, but that species was my favorite.
Gobies dig to find all the fauna they can...so if your rock is not good and stable, they will cause it to topple. The constant sifting does turn the sand, and keep it looking fantastic and clean. The bad thing is that they feed on all the fauna from the sand bed, and then starve... unless they have found a way to eat frozen food. Don't count on it, most do not eat anything but what they find in the sand. I lost a golden headed sifter that way...it was terrible to watch, and once it's little tummy sucked in, it couldn't eat if it wanted to. I liked it so much, that I seeded the sand and after waiting for it all to mature, got another one. In the process of sifting, it also got some frozen Mysis that had settled on the bottom of the tank to my delight!...When it was out of fauna, it fed on the Mysis, and I had it for many years until I gave it away.
Sand deeper then 2 inches may allow toxins to settle deep in the sand bed layers, and if it ever gets disturbed, (Moving rocks and re-aquascaping, for example) it could release those toxins into the tank, and kill everything. Some SW critters need a deep sand bed, such as a wrasse...but as they do their thing, they also move the sand so it doesn't get toxins trapped in the layers.
