No one has approached the LED or Xenon lighting possibilities in this thread yet. (that, or I missed it).
I have had Metal Halide lighting in the past and liked it a lot... after you got past it's major drawbacks.
1. Extreme heat that needs to be compensated for.
2. High monthly energy bills to run the ballasts.
I've been reading and doing my homework on lighting for my upcoming Reef tank. So far I am leaning towards something like the LED solution brought up by Aquarium-LED, and then supplementing the blue spectrums with T5 flourescents. My tank is 8 ft long but will only be 15" tall, so this is a really viable approach for me. Metal Halide also brings up the added problem of too much evaporation in a tank with this surface area to water volume ratio.
I've read up about various Xenon DIY trials and think they have a ways to go yet before that is viable, but I'm becoming more and more of a believer in LED technology.
The current drawback to it is the heat burning out the bulbs. Anyone who has chucked out 4000$ for a Polaris system and then had 1/2 of it fail in 6 months knows what I am talking about. But that is what makes the Aquarium LED choice interesting and viable. I think they have the right idea so far although I would like to improve even more on their design and add some type of active fan cooling to their model. Most likely this would involve changing the heat sink to be something flatter than their bulb-like solution.
Ah, now I gotta go find the link from these forums. ZapconJ was testing 2 of these units out and seemed to be having good luck with them. They don't currently offer anything in the blue range. Just whites. 10-14k I think, and ~370.00 per 'bulb'.
Bah, lemme go find links! I shall return.
I wanna talk more about LED's as a viable alternative to Metal Halides though. Not only can I not afford to run the Halides, but I think they do many things that would cause as many problems as they solved on my particular tank.
Is there a good reason anyone knows of that makes Metal Halide superior to LED's ?