Aqualight Melted

[Saltair]

I am looking for some input on my lighting problem. I was using a Coralife Aqualight with their T5 18W 6700K and Colormax bulbs. It fit on my stock plastic hood reasonably well, and my plants seemed to respond to it pretty well. Last night I returned home to find the fixture flickering and discovered that one bulb was out and had melted the plastic lens cover that was over it. I had researched whether this fixture should be used over a hood and indications were that it could and should be. Was I wrong? Was this an isolated failure or was I using it wrong? I am looking into replacement with either the same fixture or something different. Does anybody have suggestions for getting moderate light into a 29gal with the stock hood on? My fish selection and other environmental variables requires the tank have a hood. I was looking at the Nova on Dr. Foss, but it appears to large to fit over my plastic hood.