You can use the computer fans. Just check the wattage of the fans you get and space available for mounting. You could also wire a external fan with a transformer. My concern also is with the fixture. You might be better off to build one. BTW the coral life pro 150watt hqi should be equivilent in output to a normal 175watt MH giving you 350 watts. Check
http://www.esuweb.com/ coral lifes website.
Doug