Help mounting MH lights

[SquidHC]

Alright so I got a pair of 250w MH lights today and have a few questions.

1. What is the ideal height above the water that I should mount the bulbs? I was planning on building a hood that is about 12" high inside. That would put them about 13.5" above the water.

2. What is the best way to protect this type of fixture from getting any water or salt creep on them? I was concerned about them melting an acrylic sheet. Should I just use normal tempered glass? Keep in mind I dont want to put a lid on the tank itself because of oxygenation so any guard would be suspended.

Heres a pic of the fixture.
http://i12.photobucket.com/albums/a201/SquidHC/lights.jpg

Thanks in advance.

[GreyHawk1968]

My 250's are 8 inches above the water. A lot of it depends on the ventalation you give them. I have two varible speed fans that blow air across the bulbs. They are mounted in a canopy and it raises the water temp by a degree.
If you keep splash to nothing you shouldn't need to protect the bulbs from anything but if you have to put something on them for your own peace then use tempered glass. Just rememeber to keep it clean or you will lose spectral efficency.