Okay this is all just me being curious
I have a spare 70 watt one of these http://www.lightingcentre.com/floodl...ight-ip65.html sitting around, I was thinking of putting a 70 watt bulb in it and building it into a hood for the tank, with two cooling fans to keep it cool.

Know I know it would depend on the size of the tank for the watts and what corals I would want.

But it more would it work ???
, I have never used Metal Halides only used T5s so would this work for marine corals

http://www.bltdirect.com/products.ph...uarium+70+watt

It will only work if the bulb has the right lighting for coral photosynthesis. This is not quite the same as terrestrial photosynthesis, it's more blue because the blue light penetrates the ocean better.
Generally if a bulb does not say it's designed for corals then it won't work very well at all.

You also got salt creep to worry about, the link you gave is broken but if the MH unit is mild steel or any other normal metal then the risk of corrosion and electrical shorting is very high. If it's not water proof then there is also a significant risk or electrocution as salt-water makes an excellent conductor and the constant evaporation from a marine tank will contain just enough salt to get into all the electronics. (Although we say only the water evaporates a small amount of salts are dragged up with it and are deposited on the first cold object they find. Over time builds up.)

Probably not the answer you hoped for but I hope it's helpful none the less.

Thanks Egalec, I knew there was a reason for them not being used or more people would have done it, I was just curious why and what problems using one would cause.