what is 50 Halo and 422w blue?
If you mean halide, then you probably mean 150w which is DE, which would mean that with it's protective lense, you could keep it closer to the water's surface, 4-6" closest. PCs and VHO I don't think it matters much as long as they're a reasonably safe distance away, 4-6" too?
Need better idea of your lighting specs.