[canadawest]

In a perfect world, it is recommended that MH lamps run perpendicular to the length of the tank as more light spreads from the sides of the lamps than does from the front and back. (Which makes sense as the back is facing the mogule socket, duh) You are effectively lighting more area of your tank by having the light spread out towards the sides, rather than shooting out the front and back of the tank.

But in most of our "not-so-perfect" worlds, this would take up precious room under our hoods for florescent tubes.

So I run my MH lamps parallel to my VHO tubes, because that way I can still have 4x48" tubes under there with the 250W MH lamp. Otherwise I would only be able to fit 2x48" tubes if I ran my MH perpendicular. I realize that I am limiting the area the MH directly illuminates to the front to back area underneath it, rather than a larger side to side area, but such is life.

For reference, my canopy has a 60"x20" area. A MH lamp and socket is almost 12" in length, so if I were to run it perpendicular in my hood that would only leave 4" in front of and behind it for florescent tubes. By running it parallel, I can maximize the 20" depth of my canopy by fitting 4x48" tubes as well as the 250W MH lamp.