Hydor recommends not using them on wavemaker for this reason...if you do, make sure the sycle is a long cycle of at least a few minutes. This is a common problem on all AC mag driven pumps; half the time they'll start backwards and clank when beginning until they return to the correct way. Hydor is trying to resolve this with a new wavemaker that continues to spin, but at very low speed so when it goes back up, it doesn't have to restart...they have yet to do so, though. Smae reason on the maxijet mod units, they put in an acrylic stopper in case it begins to spin the wrong way.