Water Parameters and Aquariums Part 2

The previous article talked about some of the most basic components- temperature, Salinity, PH, nitrate, nitrite, and ammonia-when it comes to water parameters in an aquarium. This article will continue to build on those. While parameters discussed in the previous article are applicable to every tank (with the exception being salinity), the parameters in this article are geared more towards saltwater tanks, particularly FOWLR (Fish Only With Live Rock) and coral reef tanks. With more complex systems there are more factors that come into play and should be tested and looked at, but this article will go over the following parameters - alkalinity, calcium, magnesium, phosphate, and TDS.

The frequency with which each of these parameters is tested will vary not only depending on the parameter, but also depending on the species of corals being kept. Some species of coral and invertebrates are very sensitive to certain parameters and as such they should be tested more regularly, so make sure to do research before adding any species to a tank.

Alkalinity (alk)

Alkalinity descriptions can be complex and filled with chemistry equations, but you don’t have to learn all of that to understand the basics about it. Alkalinity plays an important role in aquariums, but it is especially important in reef tanks. Simply put, alkalinity is the ability of water to hold its pH. It acts as a stabilizer and without it, reefers can experience swings in pH (which comes with its own set of issues), corals melting, bleaching, and limited growth. Corals and other invertebrates living in a coral reef utilize minerals in the water to form their exoskeletons and if the alkalinity is too low they are unable.  Knowing what a tank’s alk is, requires regular water testing. When it comes to testing it can be rather confusing as different test kits use different units to measure. There are 3 standard units that you might see in test kits- ppm (parts per million), carbonate hardness (dKH and KH), and meq/L (milliequivalents per Liter). The tank’s alkalinity should range between 142-215 ppm/ 8-12dkH or KH/ or 2.86-4.29 meq/L. If for some reason you need to convert from one unit to another, do a web search for “Alkalinity conversion” which should provide resources you can use.

Testing the alkalinity or KH of a tank is easy and is typically done using a method of testing called titration. This is where drops of a liquid are added to a mixture of tank water and dye until the mixture changes and holds its color. As with other water parameters that are tested in reef tanks, there are a variety of test kits that can be used, from ones that are done manually, to ones that are automated. The manual test kits which you have to read the values yourself are ones like the Salifert, RedSea, and API. The next step up from the kits are products like the Hanna Checkers where you will add the sample, mix with the powder and the checker unit will give you an alkalinity reading. Finally there are high powered test units like the handheld Hach testers with the probes which monitor alkalinity and provide that data to the device. Lastly, there is another type of test- the dip strips- that can be used but when it comes to alkalinity and other parameters, it is best to invest a little bit more money and stick with something that is slightly more accurate.

Calcium (cal)

Similar to Alkalinity, calcium is important in all tanks, but it is even more important when it comes to tanks that have inverts and corals. All corals and invertebrates utilize calcium in the formation of their exoskeletons and their shells. As they use it, it will become depleted from the water in the tank, but without testing it is hard to know where the calcium level is at. Calcium will be measured in ppm or parts per million and the target values for a reef tank are between 380 and 450. Depending on who you talk to, what your goals are, and species you are keeping, it may be even higher. The amount of calcium in the water is tied to the amount of magnesium as together they are a measure of total hardness. If the calcium in a tank becomes depleted, there are several ways to replenish it. The first and easiest way is by doing a water change. This will replenish all the minerals and trace elements in the water. Different brands of salt will have different minerals and elements, so talk to your LFS about which brand is best for your tank. The other way to replenish calcium is through dosing. Dosing can be done using a 2 part solution such as Reef Fusion 1&2 by Seachem which targets several different parameters. It can also be done by slowly adding Kalkwasser to the tank, or lastly, it can be dosed using a calcium reactor. 

The way to test for calcium can be done manually using self read test kits, manual kits that provide digital readouts, and tests that are done by high end aquarium water chemistry testing which provide a more in-depth readout and information. Manual test kits can be purchased individually, however some, like the API Master kits will come with tests for multiple parameters. Many of the kits, like the alkalinity tests, utilize titrations and match the number of drops needed to make the color of the mixture change. The number of required drops is then compared to charts and a calcium value given. The test that provides a digital readout, will still require you to add water and mixture to a test tube before it provides any data. Lastly are the in-depth water chemistry analysis like the ICP tests where a sample is sent off to a company and a detailed report is sent back after several days.The do it yourself titration test kits are affordable, accurate, relatively easy to read, and are a really good option.

Magnesium (mg)

While on the surface it might not seem like magnesium (Mg) is all that important, but it actually plays a large role in saltwater. The magnesium level of a tank’s water is tied to its calcium (together they are a measure of total hardness). Magnesium ions in the saltwater will bond with carbonate ions this in turn allows for calcium to be added to the water without the alkalinity dropping. If a tank’s water chemistry is in line, you should be able to calculate a tank’s magnesium level from it’s calcium level, by multiplying the calcium value by 3. When testing, magnesium values will be given in parts per million (ppm) and should range between 1300 and 1400. Certain mixes of salt have different ratios of elements and minerals in them and some, like Reef Crystals made by Instant Ocean have high levels of magnesium, so it is important to be mindful of this when choosing a salt for your reef tank.

It is impossible to know what a reef's water parameters are without testing. The sensitivity of the species you are keeping, should determine the frequency that magnesium should be tested, though as a rule of thumb, calcium and magnesium should be tested at the same time. When it does come time for testing, magnesium is one of the commonly tested reef aquarium parameters and there are several different options for testing which range from around $15 all the way up to a few hundred dollars. Each of the options come with enough materials for multiple tests to be run. On the high cost end of the spectrum are the commercial grade water chemistry testing machines that are very accurate and provide a digital readout within minutes of getting the sample. Slightly more affordable are the tests like the ICP Analysis where a sample of tank water is sent out and analyzed by a commercial laboratory. Still more affordable and just as accurate are the digital checkers made by Hanna, which provide an easy to read value. Lastly are the diy test kits where you collect the water sample, add liquids or powders, and then compare a value to a chart that gives a corresponding magnesium value. Unless a reef tank is having an issue that needs fine tuning with respect to its minerals and trace elements, the best option for magnesium testing (due to cost, ease of testing, and accuracy) would be one of the digital checkers (Hanna) or the diy test kits (Salifert orASF).


Phosphate, written as PO4 can be one of the peskiest water parameters, because of all the issues that are associated with it when it is elevated. In a tank, phosphate can come from a variety of sources such as excess food, fish waste, and other excess organics. It can also come from the mix used to make the water and even from the water itself. Out of all the different water parameters reefers and aquarium hobbyists work with, they probably spend the most time struggling with phosphates and trying to get them in line. Like other parameters such as nitrate, phosphates can be reduced by doing water changes, but the side effects from elevated phosphates can linger much longer. Too much P04 in a tank can lead to blooms of hair algae that rapidly takes over a tank and is difficult to remove.  It can also cause the growth of brown algae on the corals, reducing their ability to take up calcium carbonate, restricting their growth, and turning them an unsightly brown. If phosphate is really high, algae blooms can get to be so big that the algae consume and reduce the amount of available oxygen in the tank.  Phosphate isn’t one of those parameters that needs to be checked every single week if there isn’t an issue. Tests for it are typically measured in ppm or parts per million and the target for reef tanks is between .02 and .05 ppm. While a fish only tank (FOWLR) can have phosphate values of 0, reef tanks do need a little bit of phosphate to thrive.

Testing for phosphate can be tricky. Oftentimes the test kits for it involved a color change that can be hard to read. Since phosphate testing is fairly limited to saltwater aquariums, there isn’t as large of a variety of test kits available as there are for some of the other parameters like pH. However, similarly to the other parameters there are a number of do it yourself kits available from companies like Salifert, My ASF, and API. Those kits are affordable but are only accurate to a certain degree. The digital testers from companies like Hanna, Milwaukee, and Hach range in cost from around $50 all the way up to several thousands of dollars. If you have a hard time reading color changes, and you have the budget for it, consider opting for the digital testers for phosphate.


The last water quality parameter is TDS. TDS stands for total dissolved solids and is a measurement that is taken on an RODI unit. When an RO maker has brand new filters that have been broken in, the TDS reading of the water coming out of the unit should be relatively low if not 0. Over time, as the dissolved solids are filtereds, the filters will become clogged and some of the dissolved solids will end up passing through the filter. At this point the reading on the TDS meter will gradually increase. The TDS meter is an excellent tool to use that allows you to determine when it is time to change out the filters. Oftentimes TDS meters are installed in line, However there are hand held units that are also available and can be used to test not only the RODI water but also tank and tap water.

These are just a few of the more common water parameters that are looked at and tested when it comes to reef tanks. The more technical tanks go even further into depth with respect to water chemistry, but for a beginner, these are some of the basics to get you started and kits that you might want to consider adding to your aquarium room supplies. If you don’t have an aquarium room yet, don’t worry give it time.


Hilary Jaffe