400 W Metal Halide Light CRAZY? or No?
 

I borrowed a 400w Metal Halide light from a friend (shop light) and I think tomorow im going to hang it above my tank as far as I can and see what the effects are, Am I totaly crazy? I have a 90G tank

The shop light most likely will be VERY low spectrum, 6K or less. So it will be very good for being a heater, as well as growing some damn nice algae. If it's higher up in spectrum then sure, if you have corals, start far away from your tank and work down to get them used to it.

There is a risk of shocking your corals due to the increase in light. It will add additional heat to the system. Your electrical consumption will increase.

You don't need 400s on a 90G.

So - Yes - it's a crazy idea. Don't do it.

I ran 2 x 400w on my 90wide, loved it. But as mentioned, it's probably not a bulb designed for tanks and would look like crap. Start slow, either higher up or reduced photo periods.