I have two 150W MH
bulbs. I initially used two PC
fans built into the hood as exhaust fans. They move 24 CFM each (for a total of 48 CFM). That didn't do the trick at all. With the lights running just a few hours the temp would rise about 4 degrees (81 to 85).
So I bought a regular 6" fan and mounted it to the back of the hood. This works perfectly. After about 12 hours of running the lights, the temp will actually drop a degree. This fan moves just under 500 CFM (that's 10 times what those PC
fans were doing).
Just thought I'd share what worked for me. The fan is made by Lasko. You can't hear it at all and it was $10 online.