MH bulb orientation question

[Delphinus]

Well, you could do that of course.

But really, like I said earlier, I think what you're referring to has more to do with the reflector. There are reflectors for parallel mounting, and reflectors for perpendicular mounting. The perpendicular ones either reflect more light into a given area, or have a focal shape that lends itself for better coverage for that of a "typical" tank.

Let me ask you this ... do you think it would matter on a cube tank?

One thought that I have, given that in your situation nothing else has changed (i.e., the surface agitation doesn't care which way the light faces), but, a halide bulb doesn't exactly emit a perfectly diffuse light coming out of it. For example, how come many pendents don't mount the bulbs vertical?

Anyhow, I assume it's a slight polarization effect. Just like with polarized lenses on sunglasses, it matters which way they're oriented. If the light waves travel parallel to a in a certain plane, and corresponding to the resulting waves/agitation orientation of the water surface, the angle of intersection will be different between a parallel mount and a perpendicular mount.

Anyhow, there are my thoughts.