OK, here is the deal:
Metal halides are for a great way to light your tank. You should plan on one bulb (and reflector, fixture and ballast) for every 2 feet of tank length. A 4' tank will need 2 bulbs, a 6' tank will need 3, etc. There are many different intensities and bulbs out on the market. I will summarize for you.
Wattages- there are basically 4 popular wattages on the market: 150, 175, 250 and 400W. Generally the intensity goes up with the wattage, the notable exception is the HQI 150DE bulb which many claim is as bright or brighter than the 250W standard, depending on which bulbs you use. More on this later. My recommendation regarding wattage is simple: Go with as much as you can afford. There is also a 1000W product on the market for extreem situations (tanks 4' and deeper.)
Ballasts- There are three types of ballasts on the market. The most common is the magnetic tar type ballast. PFO makes a good one that comes in single or dual configurations (single powers one single bulb, dual powers two.) The second type is an electronic ballast. These ballasts are more efficient than the magnetic type and in some cases the bulbs burn brighter. Many electronic ballasts must be tuned for a particular bulb, which makes them a pain if you want to change bulb manufacturers or bulb colors. They are more prone to failure, and cost quite a bit more. Some say they run cooler and lengthen bulb life but I am not convinced. The third type is the HQI DE (Double Ended) ballast which requires a special fixture and specific bulbs. These bulbs have a ton of intensity for only 150 watts, but are very expensive and have some other considerations.
Bulbs- There are probably 100+ different bulbs available. Bulbs are marketed with a rating that indicates the color of the bulb. This is referred to as color temperature, and is rated in Kelvins (A measure of temperature where 0K is absolute zero and 273, I think, is freezing.) Popular choices are 6500K, 10000K and 20000K. The higer the color temperature, the bluer the bulb.
The 6500K Iwasaki is a popular choice since it has the highest intensity within the wavelengths that are appropriate for photosynthesis. This is a hotly debated subject, but many believe that the 6500K bulb is the best for coral growth. It is also one of the least expensive of the bulbs for a given wattage.
Unfortunately, the 6500K bulb has a yellowish/green tint to it, and many aquarists don't like how the tank looks to humans. One solution is to add supplemental actinic (black light) lights to make the tank look bluer. Most use VHO or PC lights to supplement. Many believe that the actinics are also good for coral growth since they provide light in spectrums that normal bulbs do not.
The 10,000K bulb is quite popular, and many run it alone or also supplemented.
Personally I use three metal halide 400W bulbs over my 180G tank. The bulbs are 6500K Iwasakis and they are supplemented with four 96W PC actinics. My corals grow like crazy, and the color is perfect for my eyes. Some corals actually do well in less light, but SPS thrives in my tank.
Hope this helps.
Ernie
