Metal Halide lamps are members of the high-intensity discharge family of lamps, making them a powerful, compact, and efficient light source. They produce very intense lighting, with a shimmering effect that mimics natural sunlight. Therefore, these lamps are often the primary choice for reef aquariums with SPS corals & clams, and some anemones that rest on the bottom of the tank. Unlike fluorescent light, which produces light throughout the whole length of the bulb, Metal Halides produce all of their light from a single point source, or smaller bulb inside containing the Halogen gases. Metal Halides are known for their intensity, with bulbs ranging from 70 Watts up to1000 Watts. Halide lights all take a few minutes to warm up to their full operating output, which may actually be beneficial to your tank mates by not startling them due to fast changes in light intensity keeping that stress to a minimum (outside of getting an expensive LED lighting setup with their 40 minute sunrise/sunset ramp times!).
Never handle a halide bulb that has been turned off for less than an hour, as they run extremely hot. You should also always avoid handling them with your bare fingers due to oil transfer, which can weaken the glass, and may cause it to burn out prematurely. The bulbs may be expensive to replace, but mark your calendar to nine or ten months. That's when these bulbs slowly turn into what you call the redder spectrum zone, which is the ideal spectrum for nuisance algae to grow. The most common spectrums used on a reef tank are from 10,000 K to 20,000 K (K stands for Kelvin, and is an “absolute temperature” scale which directly relates to the lights “color”). 6,500 K bulbs are good for refugiums and macro algae, but not for a reef tank. Never buy a 6,500 K Halide bulb for your reef, no matter how cheap it is!
The Metal Halides produce intense heat, which may cause your tank to rise in temperature. It's recommended to have fans, fixtures to raise the light system higher from the water, or a chiller to keep the water at the ideal temperature. A single bulb will cover a four square feet area so two bulbs are required if your tank is 48 inches long.
MetalHalides have two different main types of bulbs, single ended (SE) and double ended (DE). Single ended lamps have a series of reflectors and don't require a UV shield. Single ended lamps all use the same mogul base screw size, but the bulbs wattage should only be replaced with what is removed. Wattage used for single ended bulbs are generally 175, 250, 400, and 1000.
Double-ended lamps (HQI) require a sheet of glass to protect the tank inhabitants from short-wave Ultra-Violet light. Double-ended lamps have two different type of socket base. 70 watts and 150 watts bulbs use one type of socket base, whereas 250 watt and 400 watt bulbs use a different type of socket base. Wattage for double-ended bulbs are generally 70, 150, 175, 250, and 400.
Halides systems come in three types, Pulse Start, Probe Start, and HQI. Pulse Start uses an extra component to ignite an extremely high voltage pulse to start the single ended lamp. Probe Start use a third electrode inside the arc tube to start the single ended lamp at a lower voltage. Hydrargyrum Quartz Iodide (HQI) ballast will put out significant more output then Pulse Start or Probe Start, and are used on the double-ended type halide bulbs.
It's important to identify the type of bulbs you are purchasing to be sure they are compatible with your lighting system since many vendors will not provide this information for you. Probe start bulbs are not compatible with pulse start light systems and vice-verse. It will only work for a very short period of time and will eventually damage the unit and the bulb will burn out. Keep in mind that German-Made single ended bulbs are pulse start and American-Made single ended bulbs are probe start. The American National Standards Institute provides a set of codes to help identify the type of lamps. It may be difficult to find the codes on the unit for some products since they are posted on a remote area of the unit.
Probe start codes: M57 (175w), M58 (250w), M59 (400w), and M47 (1000w).
Pulse start codes: M137 (175w), M138 (250w), M135 (400w), and M141 (1000w).
HQI: M85 (70w), M81 (150w), and M80 (250w).