Compact fluorescent (aka power compact) aquarium lighting utilizes fluorescent bulbs that are bent in half. These VHO lamps deliver a higher concentration of light energy in a smaller space than straight tubes do, although the light concentration is less than with metal halides They are also different from regular fluorescent tubes in that they are single ended, as opposed to the typical double-ended construction of most other fluorescent lighting.
There are two different incompatible standards for compact fluorescent lights, the straight pin style (aka European), and the square pin style (aka Japanese or Panasonic). The straight pin connector has the four pins in a straight row while the square pin connector has the pins in a square configuration. Current USA bulbs and Coralife lights we sell are straight pin, but some are square pin. Neither standard is superior--you just have to be careful when buying replacement bulbs that you get bulbs with the correct pin configuration.
Hoods & Retrofits
If your aquarium has a canopy (usually a wood box cover) you probably want a retrofit, such as the Aqualight Retrofit or Current USA SunPaq Retrofit. Both the Aqualight and SunPaq come complete with the bulbs mounted to a reflector--you just need to screw the reflector to the canopy. The only other thing you might need to complete your lighting system is a canopy fan to dissipate the heat. You might also want to consider getting a VHO Retrofit. VHOs (Very High Output) lights are fluorescent lights also, and in many cases you can get the right amount of light more easily using a VHO Retrofit. For tanks deeper than 24 inches and tanks with hard corals you might want to consider a metal halide retrofit.
You will see bulbs listed as Actinic, 420nm Actinic, 10,000K, 6,700K, SmartPaq , 50/50--and in a variety of other ways. Before purchasing you generaly just need to know the following. Reef tanks require half the lighting to be actinic lighting and half the lighting to be full spectrum daylight lighting. For the actinics any bulb listed as an actinic--whether it is a True Actinic, 7,100K Actinic, Actinic or 420nm Actinic--will be adequate. For the daylight bulb the best bulb for corals is the 10K (aka 10,000K) bulb. 6,700K and 8,800K bulbs are also fine. In many situations, because you will only have one row of bulbs, you will to get a 50/50 bulb in order to be able to spread the actinic and full spectrum daylight lighting evenly. The 50/50 bulb has one half of a single power compact bulb actinic and the other side full spectrum daylight. The Current USA 50/50 bulb is called the SmartPaq and the Custom SeaLife 50/50 bulb is called the SmartLamp.
Freshwater planted tanks usually use the 6,700K, 6,500K or 5,500K bulbs. Fish only tanks can use any light that makes the fish look good. The fish really don't need to the light to survive--so it is really a matter of aesthetics. Most people will be happy using 6,700K bulbs, which are very close to daylight, for fish only tanks.
How much Light Do I Need?
This is a very complicated issue, but here is the general rule of thumb. For a tank with only soft corals or live rock, you'll want a lighting system with about three watts of light per gallon. For hard corals you'll need about five watts per gallon. For tanks over 24 inches you may want to consider a metal halide. Always keep in mind that the farther the light is from the coral, the less light will reach the coral. So you may want to put corals needing more light toward the top of your tank.