My clam keeps moving from one part of the tank to another (probably to get better flow). In the spot I originally placed it, the clam opens fully and seems to do fine. Yet, the next morning it is on its way to another part of the tank. Is it ok to put the clam up on a small piece of PVC to keep it from moving, or is this cruel (also, if it is not cruel to keep your clam from moving, how do you do it). Thanks.