We all know we should test our aquarium water regularly. Yet many of us fall into the "well, the tank looks good... so there probably aren't any problems" school of thought. Unfortunately this mentality often leads to major headaches down the road.

Algae blooms seem to pop up out of nowhere. Corals once vibrant and growing like weeds suddenly stop growing, exhibit less polyp extension and brown out. Fish color becomes drab and their behavior changes. These sorts of problems are often linked to water quality issues and can be avoided by simply testing your water parameters on a regular basis to make sure levels are being properly maintained.

This article will provide an overview of which parameters should be tested in a reef aquarium, how to test them and acceptable results.


Although there are different opinions on what the ideal temperature should be for a reef tank, most will agree between 78-82 degrees Fahrenheit is best. There are numerous ways to measure your aquarium's temperature, from the standard "bulb" style to digital thermometers and electronic controllers with temperature probes. I use a controller (Neptune Systems AquaController Jr.) to monitor and control my tank's temperature. Temperature controllers not only monitor your aquarium's temperature but can also be programmed to turn on and off your heater and/or chiller.

Salinity/Specific Gravity

It is important to measure how much salt is in your aquarium water. Salinity is the measure of total salts in a given weight of seawater expressed in parts per thousand (ppt). Specific gravity is used by aquarists to measure the salt content of seawater and is a dimensionless valuei . For a reef aquarium, a specific gravity of 1.025-1.026 (salinity: 34-35 ppt) is generally recommended. Most people use a hydrometer or refractometer to measure their salt levels, but electronic salinity monitors are also available.


pH is a measure of free hydrogen ions in water on a scale from 1.0 (acidic) to 14.0 (alkaline). pH is a contraction that stands for pondus Hydrogenii (weight of hydrogen). For a saltwater tank, we want to have a higher pH with a level between 8.2-8.4. Hobbyists have a few different options for reading pH levels. One of the more common options is a liquid test kit. Most test kits use a color chart to represent the value of the pH. The tester will place a pre-determined volume of tank water into a test vial, add a few drops of indicator solution and compare the change in water color to a chart with corresponding pH values. While easy to perform, comparing colors can be hard, especially if you are color blind or in a dark room. I switched to an electronic monitor to read pH in my tank. A probe measures the pH of the aquarium and the monitor provides a digital read out of the levels. Although this setup costs more money, you have the luxury and peace of mind of 24/7 pH monitoring. The results are also easier to read and the monitor itself should last for a long time (my first pH monitor is still going strong after 14 years!). pH probes should be replaced every 12-18 months.


These compounds are toxic, especially in high levels, to the livestock in your tank and should be maintained at undetectable levels (i.e. 0). Nitrite is less toxic in a saltwater aquarium than a freshwater aquarium. Nevertheless, maintaining the levels at zero is ideal. With established biological filtration, neither should be an issue. When a tank is first set up, these levels should be monitored weekly or every other day. Once the tank has fully cycled, testing less often is acceptable (once a month or every other month). Liquid or dry reagent test kits for ammonia and nitrite are the most affordable and practical way of testing these levels in the home aquarium. Like pH test kits, color chart comparisons are used to ascertain the level of ammonia and nitrite within the aquarium. Another great option for monitoring ammonia is the Seachem Ammonia Alert. This easy-to-use test allows for constant monitoring of your tanks ammonia levels. The center disc on the kit will change color if ammonia is present in your tank's water.


Nitrate is the final byproduct of the nitrogen cycle (bacteria breaks down ammonia into nitrite and then into nitrate by a different bacteria). While it is generally not considered harmful to fish, moderate to high levels can create health problems with invertebrates, even death. It can also lead to problematic algae growths within an aquarium. Keeping nitrate levels low (at or close to zero in a reef tank) is very important to overall aquarium health (learn how to reduce nitrate levels here). The most popular way to monitor nitrate in an aquarium is by using a test kit. There are also electronic monitors that display up-to-the-second nitrate levels for those who do not fancy test kits.


Like nitrates, phosphates are a fuel for algae and can inhibit a coral's ability to utilize Calcium from water. Phosphates are introduced into aquariums in numerous ways. Most commonly they are introduced from fish food, tap water or additives and supplements. Although this article does not delve into ways to reduce phosphates, I will suggest using phosphate removers and RO or RO/DI filtered water. Testing phosphate levels is commonly done with a standard test kit (liquid or dry tabs). Photometers are also available and will give you more accurate phosphate readings, even at low levels.


Calcium is the building block used by calcareous organisms to build their skeletons/skeletal structures. Many organisms, including corals, clams, feather dusters, coralline algae and other calcareous algae, like halemida, all need Calcium. Calcium also has a strong relationship with Alkalinity in the aquarium. According to Mike Paletta, "There is a strong relationship between Calcium levels and Alkalinity that should not be neglected. If the Calcium levels get high (over 500) there is a tendency for Alkalinity to drop. Conversely, if Alkalinity levels get too high, Calcium levels will tend to fall as Calcium precipitates out." Calcium levels should be kept around 350-450 parts per million (ppm). There are numerous test kits available for testing the Calcium levels of your tank water. Most use a titration method to determine the levels.

Starting color for titration (Salifert kit)

Ending color for titration (Salifert kit)



Alkalinity is the capacity of a system (your aquarium water) to resist a downward change in pH. Also known as carbonate hardness. It can be measured with two different readings, milliequivalents per liter (meq/L) and degrees of carbonate hardness (dKH). Test kits give readings in one, the other or both. A higher Alkalinity level helps deter rapid changes in pH and, as mentioned in the Calcium description, can have a direct effect on the Calcium level in the tank. Natural seawater has a value around 7 dKH (slightly lower in some areas; slightly higher in others). Most hobbyists recommend keeping the Alkalinity levels between 8-12 dKH in our home aquariums. Like Calcium test kits, there are numerous test kits available for Alkalinity as well, many using a titration method or simple color change methods to determine the Alkalinity levels.


Magnesium is the third most abundant ion found in seawater. It plays a direct role in maintaining Calcium and Alkalinity levels within the aquarium. When Magnesium levels fall too low it causes Calcium to precipitate out of the water. Alkalinity levels also drop. Maintaining Magnesium levels in your tank will help keep Calcium and Alkalinity levels in balance as well. If you are having issues with Calcium and Alkalinity levels in your aquarium and haven't checked the Magnesium, you now know where to start. Your Magnesium levels should be around 3 times what your Calcium levels are running at. In natural seawater, Magnesium levels are around 1285 ppm. Levels are less in areas where the specific gravity is lower. Testing of Magnesium levels are also done with a titration test kit.


Strontium is similar to Calcium (chemically) and is therefore utilized by corals to help build their skeletons. According to Sprung/Delbeek in their book The Reef Aquarium Vol. 3, "Anecdotal reports by aquarists link Strontium supplementation with rapid coral growth." In natural seawater, Strontium levels are around 8 ppm (meq/L). Try to maintain that level within your aquarium. Titration kits from Seachem and Salifert are available for testing Strontium levels.


I am not going to explain the differences between Iodine, Iodate, Iodide, etc. since it can become quite confusing. For a solid explanation of the various forms of Iodine, read pages 184-185 in The Reef Aquarium Vol. 3. Iodine is found in most soft corals, marine algae, gorgonians and anemone tissue and helps these organisms grow. Many people also use Iodine as a dip for newly acquired corals as a preventative. Iodine kills bacteria and can help damaged corals by killing off any bacteria that may lead to infection. Kits are available for testing Iodine from reputable brands like Red Sea, Seachem and Salifert. If you are dosing Iodine supplements, it is a good idea to test their levels to prevent an overdose.


Corals and other living organisms within our reef tanks are constantly utilizing elements from the water that surrounds them. In the ocean these elements are naturally replenished but within our aquarium we must find ways to replace them (additives and supplements, for example). Dissolved organics are also excreted from these various organisms. By testing your tank water, you can determine the proper course of action needed to maintain proper parameters. Don't neglect your aquarium by infrequent water testing. Spend an hour each week testing your water so you don't have to spend weeks or months resolving problems caused by poor water quality down the road.