slick,
I have my MH/VHO fixture resting on a frame I built out of 2x4's on top of the tank. That puts the glass which protects the bulbs about 5-6 inches above the water. I get a little splash on the glass, but not much. However, I don't have any large, active fish, either. I'd guess you'd be safe at 8 to 10 inches but also wouldn't want to lose an MH bulb or electrocute any animals so if you can, I'd find a way to protect the bulbs.
By the way........thought you were going to reduce your lighting to try to get rid of some algea??? What's with going to the MH's now? Make sure you reduce your lighting time.....drastically from the 11+ hours you're now doing. Start with a couple hours a day and work up slowly to 8 or so or your water will look like lime jello!
Jim