i have a 12g and my ballast doesnt have anything to do with my fans,,,,my fans come on when my daylight does though....so does your fans run all the time? or just when the daylights come on?....
I was thinking it can be the ballast, or just the connection to the bulb...
try puttin in new bulbs and obviously if that doesnt make it work, then check the connections,,, and if not that,,,,, its probably the ballast....(as long as all wire are okay also)