I do not have a standard metal halide canopy that puts the bulbs much higher
over the water surface.
I am afraid that 400w MH would cook something. ( water , acrylic, my
existing pc lamps ).


2" from the water with 400w MH bulbs wont be a problem.

go for it

however you can place PC lights within about 8" of a 175MH bulb.

--
Richard Reynolds