Metal halide aquarium lights are among the brightest fixtures available for aquarium systems. They are a great choice if you’re interested in keeping clams, anemones or light-loving corals like acropora and millepora. There are two main types of metal halide aquarium light bulbs: single-ended (commonly referred to as mogul based) and double-ended. Metal halide bulbs should be replaced every 9-12 months. For metal halide aquarium lighting, you will need one bulb for every two feet of tank length. Wattage depends on the depth of the aquarium. Generally speaking, 175 watt bulbs are a good choice for aquariums up to 22 inches deep. For tanks 22 to 28 inches deep, a 250 watt bulb is ideal. Tanks deeper than 28 inches require a 400 watt bulb.