My 6100's used to stirr my sand around constantly leaving only glass bottom in the middle and two huge piles of sand on the sides. I thought I was going to end up with expensive maxijets because I had to crank them down so much. Roger at Tunze said to aim the flow of each to about 1/3 of the way off of the front glass. No more sand storms and now they are set to alternate between 100% and 30%.