If you have anything that is making bubbles in the water there will be a problem the bubbles will splash water up on the bulbs. I don't see a problem if you don’t have bubbles other than the evaporation of water will eventually corrode the electrical contacts on the bulb. If the glass is hot and water gets splashed on bulb there is a chance that the bulb will break and with nothing covering the tank all the glass will go in the tank. Just a few things that could happen, I have an incandescent grow lamp over my Refugium with no glass. No problems so far just have to be careful when messing around with the tank