That's sad. We don't know the cause, but I'll say this anyway: Be careful. This is a dangerous hobby. Remember these 3 words:
GFI
GFI
GFI!
Salt spray buildup + Electricity makes even the best built fixture a timebomb. Everything plugged in around your tank needs to be monitored. Just last weekend, my wife smelled smoke (electric arcing). It was a t-5 Coralife fluorescent fixture that had a modest amount of salt buildup. The salt had leached up inside behind the plastic cover, and was shorting at one of the bulb sockets. This was arcing and had burned through the plastic housing. I heard snapping and popping and sparks were flying. Now, this was not on a GFI because I had lazily plugged it into a non-GFI because I was in a hurry one day during maintenance. I learned my lesson.
Be safe.