In order to ascertain how a reef tank is doing it is necessary to do water testing. Unfortunately the testing aspect of maintaining a marine aquarium is often overlooked. This may be due to fear of complicated tests or that a degree in chemistry is necessary in order to do a proper test. Fortunately, testing the water in a reef tank has never been easier. The test kits that are available right now are easy to use and require relatively little in terms of time or expertise. Also, the desired values of each of these measurements are now well-established. Once a test is done and the level of a substance is determined it is easy to see where it falls compared with known values and to then make adjustments as necessary.

The number of tests that need to be done on a regular (weekly) basis is not overwhelming and should not take more than 10-15 minutes per week to complete. These regular tests should include pH, salinity, alkalinity, calcium, phosphate, and nitrate. When these tests are taken, the temperature of the tank as well as the general appearance of the tank should also be noted. These tests should be done at approximately the same time each week to ensure consistency. This is necessary because some values, such as pH, can vary depending on the time of day that they are taken, so the values will only be consistent if they are taken around the same time of day. All of the results should be kept in a log so if a value of one of the measurements is changing gradually a trend can easily be spotted. In the same way, a dramatic value change of an element will stand out immediately. A retest can then be done, or a search for the cause can be initiated.

Salinity is the simplest regular test that should be done on a regular basis. It is a measure of the amount of salt dissolved per kilo of seawater. The average salinity of seawater is 39.7 ppt. Measuring the actual salt content is somewhat difficult, so in order to determine the salt content an indirect measure has been developed that uses the water's density to determine the salt content. The device used to measure this is the hydrometer. Glass hydrometer are prone to breaking, so the more common plastic box hydrometer is more frequently used. Using the density measurement, the usual recommended range is 1.022-1.025 at 25 degrees C.

Another measure that should be taken regularly is pH. pH is a measure of the amount of acid and base present in water. When these two are in equal proportions, the pH is 7 and the solution is considered to be neutral. When more base is present than acid the solution is considered alkaline while when more acid is present the solution is considered acidic. The generally recommended range for pH in a saltwater tank is 8.0 - 8.4 (+ or – 0.10).

Alkalinity, or what used to be called buffer or carbonate hardness, is another test that should be done on a regular basis. Simply put, alkalinity is a measure of the water's ability to maintain a certain pH when a given amount of acid is introduced. In a marine system, it gives an approximation of how many carbonate ions are present. The usual units of measurement for alkalinity are mEq or dKH. The normal value for seawater is 2.5 - 3.5 mEq (7 - 10 dKH). A closed system should be maintained at this level (or slightly higher). Along with alkalinity, it is also necessary to measure the calcium levels as the two are closely related. Usually when one of these measures gets too high the other will be on the low side. The level of calcium in normal seawater is 380 - 480mg/L. This (or slightly higher) is also the desired level for a marine aquarium.

All of the above measures are assessments of things that need to be added to the system over time. There are two additional tests that should be done on a regular basis to assess compounds that need to be removed from closed systems. The first of these compounds is nitrate, which is the end product of biological filtration and the nitrification process. Corals generally live in areas where nitrate concentrations are less than 0.125 mg/l. While fish and corals have successfully been maintained in tanks with nitrate levels as high as 40ppm, in the long run it is much more advantageous to keep the nitrate levels as low as possible. Levels under 5 ppm are tolerable, with levels of 1 - 2ppm or less being optimal. The other compound that needs to be tested for regularly is phosphate. On most reefs the phosphate level is under 0.02mg/l. Just like nitrate, this compound is a fertilizer for algae and for this reason alone should be kept as low as possible.

Water testing is not sexy and does not add to the beauty of a tank in the way that adding a coral or fish does. However, replacing fish or corals lost unnecessarily to poor water quality is a huge expense. So it makes sense to take preventative steps to ensure the water quality is as high as possible by doing regular water testing. Ease of use and measurement are both essential in making sure that a test will be done. If a test is too difficult to do or its accuracy too difficult to ascertain, then the odds are that the test will not get done. For these reasons, choose test kits that are simple and straight forward and that do not require a lot of time. As noted above, keeping a record of test readings will provide a valuable record showing how the conditions in the tank change or are consistent over time. All of this is essential in order to increase the likelihood of long-term success with saltwater systems.

Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers.