lighting

i was thinking about using a 400 watt high pressure sodium/mercury vapor grow lights for one of my tanks, the light is not made specifically for saltwater, its used for growing i guess plants, is that almost as good?

 

NO,
Its the color of the bulb that makes it what it is.
The reason flourescents, halides, and LEDS are used mainly is that they can be tweaked to the proper spectrum of light required for corals to survive while keeping algal growth at a minimum.
HPS=Red/yellow light
Red/yellow light=Algae.

 

You may want to stay away from any bulb that is meant for plants. They tend to produce a different wavelength then actinics or 10,000K bulbs. The plant bulb spectrum is more red which is what plant likes, the bad news is that algae will really like it, so you may have a lot of algae growth

 

to keep it simple: corals are plants and what grows on them is algae.
some makers of metal halide bulbs designed for a reef use high pressure sodium/mercury vapor bulbs but meet the desired spectrum along with other factors. what matters is par and kelvin. this is a very technical question and cannot be answered easily, heres a site:
Advanced Aquarist's Online Magazine - Feature Article: Colors by the Thousands - Light, Colors and Corals, Part I