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. This can be raised with a commercially available buffering agent or through calcium-rich substrata. Carbonate hardness (KH) should be between 8 and 12 degrees. A calibrated calcium reactor can assist in maintaining both pH and carbonate hardness. 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. Most nitrate, which is less toxic to fishes and most invertebrates than nitrites, accumulates in the water until it is physically removed by a water change. Ammonia and nitrite should be tested regularly; any detectable levels (i.e., over 0 ppm) can be indicative of a problem. Nitrates should not exceed 20ppm in reef tanks, or 40 ppm in fish-only tanks. It is normal to have a small amount of nitrate buildup, and some livestock are more capable of living in these conditions than others. Most hermatypic corals, while able to assimilate nitrate, cannot be expected to survive indefinitely with chronically high nitrate concentrations (>40 mg/L as nitrate ion (~ 10 mg/L nitrate-nitrogen)).

Other suggested tests include those for calcium, alkalinity, iodine, strontium, molybdenum, and other trace minerals. It is often beneficial (and necessary) for the aquarist to research the water chemistry parameters for the specific organism that is desired.