and are most commonly maintained at . In regions where the ambient temperature is less than the desired temperature of the aquarium this generally necessitates the use of an aquarium heater. In some areas where ambient temperature is greater than the desired temperature, or for coldwater systems, refrigeration devices known as "chillers" are used to cool the aquarium water. Water testing Marine aquarists commonly test the water in the aquarium for a variety of chemical indicators of water quality. These include: Specific gravity, a relative measure of water density, is normally maintained between 1.020 and 1.024 in aquariums with fish only, and 1.023 and 1.026 for aquariums containing invertebrates. Salinity should therefore be between 28 and 35 ppt, with the higher values being beneficial in advanced reef systems. Because salinity is by definition directly related to specific gravity, both can be tested with an inexpensive hydrometer or refractometer. pH should be maintained between 8.1 and 8.3. Ammonia should be near zero. Nitrite should be near zero. Nitrate should be well below 10 ppm, but close to zero is best. Phosphate should be below 0.3 ppm. Alkalinity should be 3.2–4.5 meq/L. or 7 and 12 degrees of carbonate hardness (dKH). Copper concentration should be measured and not rise over 0.15 ppm Calcium should be around ~ 400 ppm Magnesium levels should be ~1250 - 1350 ppm pH can be raised with a commercially available buffering agent or through calcium-rich substrates. A calibrated calcium reactor can assist in maintaining both pH and alkalinity. Using purified water from a reverse osmosis/deionization (RO/DI) unit can prevent KH and pH fluctuation. The nitrogen cycle refers to the conversion of toxic ammonia to nitrite and finally nitrate. While fish waste (urine and feces) and decaying matter release ammonia, the majority of ammonia released (approximately 60%) in both marine and freshwater aquariums is excreted directly into the water from the fishes' gills. Biological (bacterial) nitrification converts the ammonia into nitrite ions, NO2-, and then to nitrate ions, NO3-. Nitrate is readily taken up and assimilated by algae and hermatypic corals. Some nitrate is converted via an anaerobic bacterial process to free nitrogen, but this process is very difficult to maintain. In the recent past, most nitrate, which is less toxic to fishes and most invertebrates than nitrites, accumulated in the water until it was physically removed by a water change. However, many marine aquarists are now employing