[mike31154]

ATI? What type & model exactly? I did a quick search using ATI dimmable aquarium light as criteria & seems these are T5HO. If that's the case, then the 'dimmable' part is very likely part of the issue. Fluorescent lighting & dimming have never been a very feasible or reliable technology. Flourescents, after the 'start' phase, generally like to run at a constant, predermined current using a specific ballast & messing with that reduces their efficiency & life. The added complexity of components required to dim fluorescents is another reason to stay away from them. Metal Halides are the same. If you want dimming, the most robust, easiest & cost effective solution is the good old filament bulb (soon to be outlawed) or, gasp, LED. The last fixture I had used a combo of MH & T5HO. Instead of dimming the T5HOs on that, I actually overdrove them with Workhorse ballasts, no dimming.

I feel your pain & T5HO will be around a little longer, but best to stick with standard, non dimmable fluorescents until they too go the way of the dodo bird.

EDIT: And for a company that manufactures fixtures for aquariums to think the problem might be humidity, is cause to raise an eyebrow, no?