Did you figure out what the problem was?
I have the same (or similar) symptom. When I first turned on my aqualight it worked fine. On the second day one of the bulbs blew. Marine Depot sent me a replacement bulb figuring the bulb was defective. The replacement bulb did the same thing, ran for 12 hours, turned off for the night, and it blew immediately when I tried to switch on the next morning. A bit more detail, in case it helps. There are two actinics which are on the same on/off switch. The one on the right blows forcing the left one to turn off. If I pull out the blown right bulb, then I can turn on the and run with just the left bulb. I can even move the good bulb from the left to the right side and it works fine. Only when I populate both then the right side blows out. Any ideas what's going on would be appreciated?