I'm using 2 MP40's on my 6 foot tank right now, on Reef Crest, though I have the Reef Crest set to just a little less than half on intensity, simply because of the power they put out and the fact right now I have a lot of low/moderate flow coral. Almost feel as if I should've gone with 1 MP10 &1 MP40, but I'm glad I got them. They're great.