The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.
You can't use Mercury Vapor Bulbs they are very yellow and would make your tank look like crap. And also the spectrum of light alone wouldn't be very good for a tank.
It would be a lot of fun trying to find a bulb that matched the size and spec you would be looking for. I'm assuming you're referring to the construction type halide lights that have the small (half-inch thick, 6" long or longer) double ended bulbs? These are designed to produce as much light, with as little power as possible -- not to provide a specific light band for your fish tank.
However... if you can figure out the style bulb that is, you can find someone who supplies various spectrums/gasses in that bulb.
The lights i am talkin about are the ones that take a screw in bulb. They sell them at wall-mart. On the box they say mercury vapor 7,000 lumens Outdoor security lights. Comes with a 6 or 8 in bulb that is a screw in type.
I've got a mercury vapor light (the one's at walmart are junk, btw) on a pole light not too far from the front corner of my driveway. It's ultra-bright and very very hot.
I used a MV on my tank for a while (not outdoor security but a plant growth one, single ended screw in) - completely useless, bright yellow and horrible brown and red algae all over the tank, scrapped it and bought a MH.
If you doa search on Google , there is a site that uses the ballast and parts from that light with a 250 watt MH bulb ( Iwasaki i think) .Says DIY MH lighting for under $100.