mstump, for one thing fluorescent lights (atleast normal output bulbs) arent hot enough to bust if they get wet, secondly the water really shouldnt splash up unless a fish happens to flop at the top of the tank. Also alot of people use metal halide bulbs that HAVE to be a good distance from the top of the water for heat issues, so for many tanks that not really even an issue. In regard to the dust question many, if not most people have a canopy that hides the lights and covers the tank even though they dont have a "top" on the tank. The canopy allows the tank to breath but still keeps stuff from falling in or settling into the tank.