Water Chemistry

By far water chemistry is the most complex aspect of saltwater aquariums and at times might seem daunting for new hobbyist. There are basic kits to measure more common molecules and ions while other kits that measure certain trace elements. It is important to note that many of these molecules are part of cycles and some interact with one another. Some important concept of cycles, ions and pH are lightly grazed below but it should be noted that completeness of these subjects are beyond the scope of this section. At the end of this section there are links to articles that provided further reading.


Nitrate cycle. The nitrate cycle include in its pathways three important molecules: ammonia (NH³ or NH³+4), nitrite (NO²), and nitrate (NO³). The nitrate cycle is the cycle that occurs when nutrients are broken down. In a well balanced aquarium and after cycling (completion of the nitrate cycle’s equilibrium) ammonia should not be detectable, nitrite should not be detectable and nitrate should be preferably less than 0.2 ppm. Many hobbyist keep a regular check on nitrate levels.

Alkalinity, Calcium and Magnesium. Alkalinity, calcium and magnesium are important for the metabolism and skeletal growth of corals.

Salinity. Salinity is measured usually in specific gravity or part per thousand (ppt). This measurement theoretical should be the measurement of all dissolved molecules in saltwater. It varies with temeprature and types of molecules that are in solution. The usual measurement in saltwater aquariums have a specific gravity of 1.025 and 35 ppt.

pH. pH is a frequently misunderstood concept in saltwater aquarium maintenance. pH is an indicator of the overall balance of several ions in the saltwater, and sometimes requires several changes for it to obtain the desired pH. Therefore, buffers that are sold are recommended only as temporary fix. An irregular measurement in pH can indicate a change or imbalance of any of the many reactions going in a saltwater aquarium. It should be noted that it is normal to see a drop in pH from day to night due to respiration and other reactions.

Trace elements. When further control is required and desired there a several trace elements that can be measured and adjusted. These include phosphate, silica, iodine, strontium, ORP, boron, iron.

Below is a a list of common measurements in the saltwater aquariums, some variance is tolerated and sometimes desired but research is suggested before changing any of these ions directly

pH 7.9-8.4
Salinity sg = 1.025 or 35ppt
Alkalinity 2.5-3.5 meq/L or 7-11dKH
Calcium 380-450 ppm
Magnesium 1250-1350 ppm
Phosphate Ammonia Nitrate Nitrite undetectable
Ammonia undetectable
Silica < 2 ppm
Strontium 5-15 ppm
Boron Iron undetectable