this might be a dumb question, but i'm gonna ask anyway.
could you use an automotive hid bulb to light an aquarium. i don't know how may of you have noticed, but those things are wicked bright, and you can get them in in just about any K temp you want. you could mount it like a mini MH pendant. all you'd have to get is a 12v source, ballast, and some sort of reflector...
I googled this and the thing that stands out it that they are very expensive! So, just as you can make a good light with LED's, there is no doubt in my mind that you can also make a good light with HID lights. But, in both cases you would be spending a lot more money than a T5 light of comparable intensity would cost.
I wonder about the life of the bulb. Aquarium bulbs are on every day for 8+ hours, car headlights for most people are occasionally on. And if they're an expensive bulb, it might not be economical to replace it frequently.
But experimentation is how great discoveries are made.
xenon gas hid bulbs are designed to last the life time of the car in most cases. there is no fillament(sp?) just a pocket of gas inside the bulb. they do tend to move up in the K range after a few years though. most are rated at 3000 hours.
as far as the heat issue goes, imagine a bulb smaller than a regular headlight bulb suspended over your tank. you can also get a pair of 35w bulbs/ballasts (around 3000 lumins each) for around $100 bucks. even if you had to replace the the bulbs once a year thats only like $45 year for bulbs. how much is one new hqi bulb and how long does it last?
i have no idea here - how much light (lumins) do you get from one 150w hqi setup? and how long does it last?
I'm aware of the cost differential, but when it's all said and done, it's easier and likely cheaper to just pick up a MH (I'm referring to pendants).
The balasts in automotive applications can get very hot. All do not handle moisture well. They draw LOTS of current during warmup (can reach >20A in some cases). My main concern would be finding a suitable reflector. That's a lot of light coming from a very small bulb. Getting it to diffuse over the area of a tank would be another issue.