I have an AGA 75 gallon tank, an Oceanic 75/105 oak cap, and six 36" VHOs under the cap. The bulbs are an inch or two above the water. I have room for two 3" diameter fans, one at each end of the cap, but I removed them because they were so noisy. Instead, I am cutting large holes in the sides and top of the hood and installing vent covers over the holes to provide more air circulation. In addition, I may hook up the hood to a silent bathroom-type exhaust fan mounted in the closet adjacent the tank if necessary. I went with 36" bulbs because I wanted room for two Sea Swirls at each end. Otherwise, I would have used 48". HTH.