I run two four inch fans with three 4 1/2" holes in the back and screens covering the holes so no one jumps out. One fan runs 24/7 the other is controlled to turn on at 76 and turn off at 75 degrees. I have 3 - 250w MH and 1 160w VHO and my average temp is 76. The room temp is 75, low temp for aquarium is 75 and high is 77.8. My stand is closed in the back with several 4 1/2" holes drilled with one 4" fan running 24/7 to bring in fresh air down low and escaping through holes drilled on the upper side of the back of the stand. I do go through 2 1/2 gallons of water a day on my 300 on evaporation which helps keep the tank cool.