Stability is a major key to success in a reef aquarium. Regularly testing your water parameters is one of the most important things you can do to ensure tank conditions remain stable
The most important conditions to monitor are:
Temperature: 78 to 82 degrees Fahrenheit is generally considered ideal temperature conditions for a reef aquarium. There are many ways to test and monitor temperature. Electronic controllers and digital or liquid thermometers are most common.
Salinity/specific gravity: put simply, salinity is how much salt is in your water. For a reef aquarium, specific gravity between 1.025 and 1.026 (34-35 ppt salinity) is recommended. Use a hydrometer, refractometer and/or electronic monitor to measure your salinity.
pH: Measured on a scale from 1.0 (acidic) to 14.0 (alkaline), pH is the measurement of free hydrogen ions in water. The pH in a reef aquarium should be between 8.2 and 8.4. Liquid or dry tab test kits and electronic monitors are available for recording pH.
Ammonia/Nitrite: Both are toxic, especially ammonia, to aquarium livestock. These parameters should be kept at undetectable levels (0) and monitored using liquid or dry reagent test kits.
Nitrate: While it is generally not considered harmful to fish, moderate to high levels can cause significant problems with invertebrate and lead to problematic algae growth. Keep nitrate levels as closes to zero as possible. Use liquid or dry lab test kits and electronic monitors to keep a watchful eye over nitrate levels.
Phosphate: Phosphate is another fuel for algae and can also inhibit the ability of corals to utilize Calcium in water. Testing phosphate is usually done with a standard test kit (liquid or dry labs), but Photometers provide more accurate results.
Calcium: Calcium is the building block that comprises calcareous organisms in reef tanks. Calcium levels should be kept around 350 to 450 parts per million (ppm). Numerous test kits are available for measuring Calcium. Most use a titration method to determine Calcium levels.
Alkalinity: Higher alkalinity deters rapid pH changes and can directly effect Calcium levels. Most reef aquarium hobbyist recommend alkalinity levels between 8 and 12 dKH. Measurement is most commonly done using inexpensive titration of color change test kits.
Magnesium: Magnesium levels should be about 3 times what your Calcium levels are. In natural sea water, Magnesium levels are around 1285 ppm. Testing Magnesium is also done using a titration test kit.