I had a sea cucumber for a long time. It did a good job in the sand, but would take occassional detours over the rock and knock things over. They are light enough though, that even as clumsy as they are, they can't knock over much if it is reasonably secured.
They are interesting critters, taking the sand in the front end and leaving a trail of "cleaned" sand behind. They sort of shoot it out the back like little bullets...
The other down side is that when it finally died it made a real mess, even though I got it removed quickly. The fibrous residue it left after it died was enough to make me stay away from them since.