Metal halide lamps are high-intensity discharge (HID) lamps. They were originally created in the 1960s but only in the last two decades manufacturers produced bulbs specifically designed to grow coral. They produce high light output for their size, making them a compact, powerful, and a somewhat efficient light source. There are numerous ballasts designed for specific bulbs. Most Electronic Ballasts will fire most commercially available bulbs designed for the reef tank. Ballast wattage must be equal to the bulbs wattage. The bulbs and fixtures come in two types, Double Ended and Single Ended Mogul Base. Double ended bulbs need to have a UV glass shield on the fixture. Single Ended Mogul Base Bulbs do not need an UV shield.
Single Ended Bulb
Double Ended Bulb
There are hundreds of Bulbs to choose from. Generally you get what you pay for. We highly recommend Ushio, XM and Radium bulbs as they have the best balance between price and quality. Generally speaking a bulb with a kelvin of 10K will give you more growth and a bulb with 20K generally beings out better coral colour. Depending on the lighting the same coral can have very different colours. Growth is often a direct result of the bulbs PAR (Photosynthetically Active Radiation) rating. Generally bulbs with a lower kelvin 6500K to 12000K tend to have a higher PAR rating. Our advice is if you want your tank to grow quickly generally a lower kelvin bulb is best. If you want you tank to show its full range of colours a higher kelvin bulb helps.
For more info on aquarium lighting click on the following links.