The aragonite-based sand in your aquarium might actually be fish poop. In the wild, parrotfish eat corals, eventually excreting the indigestible portions as sand. This sand can accumulate and actually form small islands, or at least add to them. The sand being excreted can also cover corals for a short time, which might cause lightening of the coral’s color or even tissue loss. Fortunately, the sandy poop is very fine and will eventually blow away.