brighter, more powerful light are commonly used as are the older VHO (Very High Output) fluorescent tubes and compact fluorescent lighting. Another light source in reef aquariums is metal halide lighting. These high output lights closely recreate the shimmering effect of bright tropical sun over a patch of coral reef. Metal halide bulbs come in a variety of spectra from 5000k on up to 20,000k and allow for a variety of light-hungry corals to survive under them. Many aquarists will use metal halide bulbs in conjunction with either power compact fluorescents or T5 fluorescents since halide lighting does not have the overall balanced effect of other light types. While very useful to reef aquarium keepers, halide lighting also uses a lot of electricity (150-400 watts being common) and produces copious amounts of heat. Most reef keepers mount halide bulbs at least one foot above their tank and some, due to the lights' heat output have had to add chillers to keep aquarium water cool. Many light fixtures come as all-in-one units with light for both daylight hours and nighttime viewing. LED lamps of 3/4 to 2 watts can be implemented to come on at night, simulating the glow of the moon over the tank. All in one fixtures require large fans to cool the bulbs and achieve maximum light efficiency. LED lighting is also becoming more common for not only lunar lighting but also to simulate daylight conditions, LEDs have a higher initial cost than other lighting sources but utilize much less energy. They are also dimmable. LEDs produce a very narrow color spectrum that is limited to a narrow band of wavelengths. For this reason an array of different colored LEDs is required to be used in combination to simulate spectral coverage that is suitable for coral growth. Early adopters of pure LED based systems have reported that the limited color spectrum provided by LEDs may not produce optimal coral growth. Notes Fishkeeping