While rarely seen, horseshoe crabs are incredibly poor at manuevering above the sand. Everytime they appear above the sand base, they end up being pushed around by my powerheads for a while before ultimately getting stuck on some intake. They are so poor at moving, they even get stuck on the intake for my overflow (which does not have much pressure). I seem to always be rescuing them from some problematic situation. Let's just say I won't be getting anymore of them. If you do want them, reeftopia.com has them quite cheap.