light fixture broken? Help please!

I got a second hand coralife 65 watt PC fixture near the beginning of the year. It is(according to the date on it) about 2 years old. I got a bulb for it around. . . the end of March, I believe and since then I have burned out 2 of them. The bulbs were new(supposedly) coralife 65 watt 10K bulbs from petsmart, the hood has an acrylic sheet on it to protect the bulb and wiring from water splashing into it/evaporation condensing in it and it isn't in a hood, so the outside air can cool it(tank is going to be covered with egg crate, with the light sitting on top of it) Any ideas why that is happening?

edit: also, my usual light cycle is around 8 hours, so no way they should have been just run into the ground.