GSMguy asnwered this perfectly. I know some people who have to run them at 12v and others run them at 6v. If your ambient temp is around 70-76F, 9v seems to be best.
Exactly. In my last apartment we didn't have AC here in Utah. During the summer months and constant 95-100F days, I was very tempted to turn my fans up to 12v. I already had a large fan blowing on the tank and fixture though to keep the tank below 81F.
The hotter a bulb gets, the more pressure inside the tube. More heat = less output. Especially when you are using the correct ballasts like ATI does. When the ballasts recognize the added resistance, they don't run the bulbs as hard. You can also overcool them as well and have the same issue. The ballasts will cut some power to help try and protect things. I don't think the ballast knows whether the bulbs are under cooled over over cooled, it just knows the resistance isn't within spec. Therefore, something could be wrong and the ballast goes into a "safety" mode.
If your room is right around 68F all the time, 7.5v shouldn't be too much of an issue. When I was exhaustively testing the different fan speeds I was in a basement where the air was about 64F all the time. I found that 7.5v worked best in that setting but as soon as the fixture was moved up into the 70F house, 9v was a better setting.
ATI used to suggest 4v until they had a slew of bad units showing up. Ballasts were failing at a rampant rate (at least on this forum) and the cause was overheating. I believe ATI suggests 7.5v now but there are too many factors to make a blanket statement like that.
Here is a great thread if you want to know more.
http://www.reefcentral.com/forums/showthread.php?threadid=1641235
Starting around page 15 is the talk about what temp the air should be exiting the fixture.