400 W Metal Halide Light CRAZY? or No?

[abcha0s]

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.

[Aquattro]

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.