Some aquarists use industrial fans on the roof to cool down the reef tanks and aquariums inside the room equally. However, it's not a wise thing to do so, especially if you have prepared an open ecosystem for acrobatic fish species. If you're using fans directly on the ceilings, at least make sure there's ample distance between the tanks and the ceilings. Otherwise, nimble and swift fish kinds can often jump up and get caught in the fan blades - resulting in their horrific death.