You should get a rotating fan that you can adjust. I hung mine upside down with a rope and a wire, so I could adjust the height. Also, the Powdery Mildew, fly about and stick around. Use some rubbing alcohol or vinegar, with a spray bottle and clean cloth. Spray it on EVERY surface and wipe, including the ceiling, and such. Also, make sure you don't have any other plants in the room. You gotta make sure your grow area is as clean as possible. Dol this before you turn on your fan. Your pots could make a difference too. If you aren't getting dry soil, where you have to water, at least every three days, then you should look into better aerated pots.
Hope that helps