got this in another thread ..hope this helps a bit with the confusion
Standard Fluorescent : Basic wattage fixtures like what comes with the tanks is standard florescent. They work great for fish only systems and even some types of reefs. If you use at least 2 quality bulbs, an AquaSun or 50/50 with a blue actinic, you can keep some mushrooms, soft corals, sponges, etc. You have to be very selective, but they can be used for the lower light corals.
VHO - Very High Output: VHO looks just like your regular standard bulbs, except with the use of special Electronic Ballasts, they can put out more wattage. An example, standard 48" bulb is 40 watts 48" VHO bulb is 110 watts. VHO has been around for many years and they have more VHO bulbs in more spectrums than any other type of lighting. This means you have a wide array of bulb choices. VHO is powerful enough to keep any coral or clam. It does not penetrate the water as well as halide, so it's best used for soft coral tanks up to 30" deep, hard coral tanks up to 24" and SPS and clams should be within 18" or less from the light. These are basic guidelines and can be broken. We personally feel VHO has the nicest look with Soft corals and LPS corals, and believe these types of corals thrive the best under VHO.
Compact Fluorescent : Also referred as Power Compacts. This is a florescent technology, except the bulb is powered from one end, not end to end like VHO. It's also a smaller U tube type design. Compacts entered the market many years ago, and on paper are superior to VHO. They have more watt for watt output. Example: 55watt Compact bulb is roughly as bright as a 95 watt VHO tube. Their lower price (on most sizes), more watt output, and compact size has made them extremely popular. The biggest drawback is the 7100K Actinic blue is not a true actinic. The lamps do not have the same glow VHO puts off, so it's a different look and may also contribute to algae problems. Until recently they only had a 6700K daylight bulb. So your bulb choices are much more limited, also limited to one manufacture for the PFO/CSL brand compacts. They recently released a 8800K crisp white and a Smart lamp which is half blue/half white. So they are making improvements in this technology everyday, a new 10K bulb is expected in August. We feel compacts work great for soft corals, LPS, and even SPS and clams. 55watt is usually great for up to 24" deep, 96watt for deeper tanks, or more clam/sps tanks. For a smaller tank, 36" length or less, it's really tough to beat compact's price.
Metal Halide: This is your most powerful form of lighting. A halide bulb creates an arc of light so bright that it can spread up to a 36" wide tank. It's the closest form of lighting compared to the sun. You can use halide for soft corals, lps, sps and clams. The most common use is for those wanting more sps and clams. Some guidelines are 1 halide bulb for every 2 feet of tank, up to 3 feet. 175watt up to 24" deep, 250watt up to 36" deep, 400watt for deeper tanks. It's very common for hard core reefers to use 400watt on even a small 30 gallon tank. When it comes to sps and clams, they can handle and thrive under intense lighting. With halide, you also need to consider the Kelvin, which is the color temp. 5500K warm white, 6500K warm white, 10K Crisp white to blue tint, 12K-20K blue tint. For mixed reefs, we recommend the 10K bulbs, for sps and clams, we recommend the 65K's. If you go 55-65K, we highly recommend VHO or PC actinics, they will improve the overall color and look of the tank.
