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
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 the use of a special section of the tank or separate tank altogether, called a "refugium." A refugium is, as its name suggests, a sheltered area that shares water with the primary, or display, tank. Refugiums usually contain a deep sand bed to allow anoxic zones to develop within them where anaerobic bacteria can convert nitrate into nitrogen gas, a useful means of nitrate removal. Various types of macroalgae can be grown and harvested from the refugium as another means of nitrate export. As refugiums become more common in marine aquaria, nitrate levels are easily manageable for even the novice hobbyist. 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 2 ppm in reef tanks, or 20 ppm in fish-only tanks. It is sometimes acceptable to have a small amount of nitrate buildup, as some livestock, especially fish, are fairly tolerant of nitrate. Most corals, while able to assimilate nitrate, cannot be expected to survive, much less thrive, with high nitrate concentrations.

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