Thanks! I guess I can try to use my old X10 Home commander to control the fan on/off since it has a temperature sensor on it.
I have a total of 656W over my tanks as well, of which, 300W are MH. In such small (relatively speaking) water surface, the water get hot quite fast. The worse is that it has only 2.2" clearance between the light and the tank. None of the clip on fans works. I was going to hook up 4 of those 50mm server ventilation fans on the edge of the tank, but after I hook them up, they are way too loud. Sounds like I have a rank mount 1U server sitting there...