Mirror

[Canadian]

Ummm...I call bull$hit on Steve's comment about diffusing MH lighting. Steve, this is one of those cases where your lack of experience in reefkeeping and minimal "research" being espoused as if it is fact, is in fact incorrect and has the serious potential of leading "newbies" astray. Yes, as Richard Harker has pointed out, highly reflective material in conjunction with MH lamps will not do anything to diffuse "hot spots." And yes, the use of a white reflector would indeed diffuse these hotspots. However, a white reflector would seriously diminish the intensity of light entering the tank (when compared to polished materials), and considering MH lighting is not as intense as we would like our lights to be (i.e. in an attempt to replicate the sun), all hobbyists are better off maximizing their intensity and secondarily paying attention to diffusion of light. Therefore, as per Sanjay Joshi's suggestions: use a quality parabolic reflector of some sort in conjunction with MH lighting to ensure that intensity is adequate.

Now, in regards to your question about using a mirror as a reflector: There are two reasons for not doing so: 1) If and or when humid air makes its way behind the mirror, it will tarnish the mirror and it will turn black. 2) Mirrored glass that close to a lamp is at serious risk of becoming extremely hot and could potentially shatter (under a variety of circumstances) due to the proximity and heat of the MH lamp. Clearly, it would not be an especially enjoyable day if you had to pick glass shards and splinters out of your reef tank. And the use of mirrored acrylic and other mirrored plastics is obviously not a possibility as they could melt and become a fire hazard.

With that said, there are a few people who have experimented with the idea of using mirrored tiles glued into their hood thus ensuring no water could contact the back of the mirror. You might want to check on RC (I believe there were a few of them there), and ask them about their experiences.