Are you running a bare bottom or sand?
On my bare bottom, i start to have vortexes or sucking air whirpools from the water surface when i have my MP 60 off the bottom of the tank, so the MP 60 is placed directly on the bottom. I found that MP40s dont haven enough umph to keep the detritus off the bottom, again running a bare bottom. To correct this issue i run one in each corner of the six foot tank, all on reef crest and some gyres up higher in the tank. But i found that i can have MP 40s higher on the glass before they start creating vortexes unlike the MP 60. Point of reference, I am talking about 23-25" deep tanks.