the technology does cause them to be relatively expensive, but these systems bring several advantages over traditional lighting. Although their initial cost is much higher, they tend to be economical in the long run because they consume less power and have far longer lifespans than other systems. Also, because LED systems are made of hundreds of very small bulbs, a microcomputer can control their output to simulate daybreak and sunset. Some systems also have the ability to simulate moonlight and the phases of the moon, as well as vary the color temperature of the light produced. Moreover, some manufacturers produce LED lighting systems in single bright and double bright intensities for sustaining coral life in marine aquariums. The choices for aquarium lighting are made complicated by variables such as color temperature, (measured in kelvins), color rendering index (CRI), photosynthetically active radiation (PAR) and lumens. Power output available to the hobbyist can range from a meager 9 W fluorescent lamp to a blinding 1000 W metal halide lamp. Lighting systems also vary in the light output produced by each bulb type—listed in order of weakest to strongest they would be: T8/12 or normal output lamps, compact fluorescent and T5 high output, VHO, and metal halide lamps. To further complicate matters, there are several types of ballasts available: electric ballast, magnetic ballast, and pulse start ballast. Heating and cooling Reef tanks are usually kept at a temperature between 25 and 28 °C (75–82 °F). Radical temperature shifts should be avoided as these can be particularly harmful to reef invertebrates and fish. Depending on the location of the tank and the conditions therein (i.e. heat/air conditioning), one may install a heater and/or a chiller for the tank. Heaters are relatively inexpensive and readily available at any local fish store. Aquarists frequently use the sump to hide unsightly equipment such as heaters. Chillers, on the other hand, are expensive and are more difficult to locate. For many aquarists, installing surface fans and running home air conditioning suffice in place of a chiller. Fans cool the tank via evaporative cooling and require more frequent top-off of the aquarium water. Water chemistry Stony corals, which are defined by their calcareous calcium carbonate skeletons (CaCO3), are the focus of many advanced reef keepers. These corals require additional attention to water chemistry, especially maintenance of stable and optimal calcium, carbonate, and pH levels. These parameters may be tracked