13 Most Essential Water Testing for Your Corals.

If you asked me what the most important factors in determining your success or failure in coral would be, I would tell you:

  1. The condition of your corals.
  2. The condition of your aquarium – water quality/water parameters.

If you take a look at the testing section of an online aquarium supply store or take a stroll down the aisle of your local fish store, you’ll notice there is an overwhelming assortment of products to help you test the quality of your aquarium water.

If you have ever been confused and wondered how to make sense of it all, you are not alone. Despite the dizzying array of testing products, rest assured that you do not need a degree in chemistry to be a successful aquarist.

In this article, I will attempt to demystify the water testing landscape and break it down to a more digestible level.


Alkalinity is a complex concept to describe. Most definitions are highly technical and include a bunch of chemistry terms that are not all that helpful for the average aquarium hobbyist. Measuring alkalinity in a saltwater aquarium helps us estimate the amount of bicarbonate available in the water. Bicarbonate is important because corals use this to make their skeletons.

White slivers in Coral

Do you see the small white slivers embedded in the body of the coral in the previous image? Even Soft Corals, like this one, need bicarbonate to make their skeletons. Alkalinity tests, also known as carbonate hardness tests (sometimes abbreviated KH), measure the amount of carbonate and bicarbonate available in the water.

General hardness (sometimes abbreviated on test kits as GH), is a measure of the calcium and magnesium available in the water. This is an important (and sometimes ignored) water parameter. You generally want to keep hardness around 10-12 kdH and do partial water changes to maintain alkalinity at the desired level.


Ammonia is bad for your corals…two kinds of bad…very bad and extremely bad. In low concentrations, it can cause chemical burns. In higher concentrations it is lethal.

The good news is that you don’t have to fight ammonia alone. Beneficial, naturally occurring bacteria will help you keep ammonia levels near zero after you cycle your tank. Always cycle your tank before adding your first corals.

Unfortunately, ammonia is created all the time as a natural biological waste product. In a healthy, mature aquarium, the biological filter should neutralize ammonia quickly – so that there is effectively no detectable ammonia (by ordinary testing standards).

On an ongoing basis (after cycling) you should keep ammonia levels at 0 ppm. There are ammonia badges you can mount in your tank that will provide an instant reading of the ammonia level in your tank, or you can test for ammonia with a reagent test kit or test strips.


Calcium is an essential element for coral health in a saltwater aquarium. Natural coral reefs tend to have calcium levels between 380-420 ppm (parts per million). For simplicity’s sake, I find 400 ppm to be a suitable value to aim for—but if you want to boost the growth of your Stony Corals; you may want to keep your calcium levels higher than 400 ppm.

Keep track of your calcium test results and watch how they change over time. This will give you a sense of how quickly the animals in your tank absorb and deplete the dissolved calcium. If the calcium demand in your aquarium outstrips a practical water change schedule, consider adding supplemental doses of calcium with kalkwasser (calcium hydroxide) or set up a calcium reactor.

Keep in mind that adding dissolved calcium solutions like kalkwasser to your aquarium water will change the pH, so be sure to dose slowly to avoid shocking any of your animals. You would generally test for calcium with a reagent test kit.


In a properly cycled aquarium, the presence of nitrate is a confirmation that your biological filter is working. Congratulations on that. If you are growing and fragging Small Polyp Stony (SPS) corals, you want to keep your nitrate levels as close to zero as possible. If you are growing and fragging Soft Coral species, a small amount of nitrate is probably a good thing. You would test for nitrate with a reagent test kit or a test strip.


Nitrite is an intermediate by-product produced by your bacterial filter. One group of beneficial, naturally occurring bacteria convert the toxic ammonia into less toxic nitrite.

Then the second group of beneficial, naturally occurring bacteria convert the nitrite into nitrate, which is relatively safe in low concentrations.

Detecting nitrite levels in your aquarium (unless you are cycling your tank) is a warning sign that something is wrong—because nitrite levels should be nearly zero as long as your biological filter is working. Nitrite is generally tested with a reagent test kit or a test strip.


pH is another one of those CCC’s: complicated chemistry concepts. pH is important because it affects how all the other chemistry happens in the tank. There are two things to watch out for, with pH; the absolute value (the actual pH) and changes in pH.

You want to have a pH between 8.1 and 8.4 for a coral aquarium. Assuming you keep your aquarium in that range, an equally important thing to remember is that you need to keep your tank from experiencing rapid swings in pH.

pH is a logarithmic scale, which means that tiny changes in the number actually mean gigantic changes in the actual chemistry of the water. Having a stable pH in the range of 8.1-8.4 is appropriate. Experiencing a swing from 8.4 to 8.1 (or vice versa) may be an issue. Most of the time, I try to keep my pH around 8.2. You can test the pH of your aquarium water with a test kit or pH probe.


In natural reefs, phosphate is present at a level of ~0.13 ppm. In your saltwater aquarium, it acts as a fertilizer for algae–because of that, I recommend you keep levels below 0.2 ppm if possible. You can get a phosphate reagent test kit at your local fish store or online.


It is pretty common in the saltwater aquarium hobby to use the terms specific gravity, density and salinity all interchangeably. If you are a chemist, each terms may have a distinct meaning to you. The takeaway from any of the three terms is essentially the same. Specific gravity, density and salinity all help determine how much salt is in the water.

You measure specific gravity with a simple, inexpensive hydrometer or a slightly more expensive refractometer. Those may sound like sophisticated pieces of equipment, but they are easy-to-use. You want to aim for a density (or specific gravity) of 1.025, to replicate sea-water.

The salinity of the ocean is actually ~ 35 g/L, but for your saltwater aquarium, it is more common to measure the specific gravity of the water as a proxy for salinity because of how easily specific gravity can be measured.

It is important to test the specific gravity routinely. Water is constantly evaporating from your reef tank. But salt does not evaporate—the only freshwater evaporates out of the saltwater. The more the water evaporates from your tank, the more concentrated the saltwater remaining in your tank becomes.

So you need to measure the specific gravity routinely to make sure you keep your water chemistry stable. Over time, aquariums tend to drift towards higher salt concentrations. So watch out for this salinity drift and guard against it by replacing the occasional portion of water from a water change with freshwater to keep your salinity in the right range.


As long as the temperature of your saltwater aquarium is in the right range (78-82 degrees Fahrenheit), keeping the temperature consistent (avoiding fluctuation) is more important than the actual temperature itself.

Testing the temperature of your water is easy. Buy a thermometer and take a look at it every once in a while. Make sure your tank stays at the temperature you set, generally 75 to 78 degrees F. Once you decide on and set an aquarium temperature, the absolute temperature (the reading) is less important than ensuring, in general, that you don’t face any major temperature swings.

One variable to consider when tracking the temperature of your aquarium is to monitor and record the temperature at different times of the day. You want to assess whether the equipment you are running (like your lights) is adding sufficient heat to warm the water to a dangerous level. Other than that, just keep an eye on the temperature to make sure nothing is wrong.

Trace Elements

Trace elements, like strontium, magnesium, iodine and potassium are important to healthy coral growth. A good reef salt will have trace levels of these elements, and partial water changes will replenish some of the elements consumed by your tank, but levels can vary over time.

Expert hobbyists will test for these elements and utilize special supplementation to maintain optimal conditions, but for most of us, you can be successful with trace elements simply by using a high-quality salt mix and staying on top of your aquarium maintenance.

Troublesome Water Parameters

The three tests below, do not need to be conducted routinely. Instead, they are generally performed only on an ‘as needed’ basis to troubleshoot a potential problem in your aquarium.


Copper is often used as a medicine in hospital tanks to kill off parasitic (and other) infections, but it should NEVER be used in your reef tank, as even trace amounts can cause big problems with invertebrates (did you notice I bolded and CAPITALIZED the word NEVER there? Just checking).

If you are having serious issues with invertebrates (corals, clams, crabs, shrimp, etc.) in your aquarium, you should test for copper to rule it out as a contaminant.

Copper, surprisingly, is found in drinking water routinely—so you never know—you may be introducing small amounts of copper into your tank without knowing it, from your water.

A properly functioning RO/DI unit should remove copper ions from the water, and a water conditioner should neutralize the copper, but the metal ion still causes problems for aquarists every year, so look for it if you have trouble.


Phosphate is a contaminant that, if left unchecked, can contribute significantly to problem algae growth. Phosphate occurs naturally during the decaying process that will occur in your tank (from food and other wastes).

If you have ever looked at a fertilizer bag (for your lawn or garden, perhaps), phosphate and nitrate levels are generally listed (or even bragged about) in the fertilizer. Fertilizer is great at one thing, boosting the growing plants.

If you have issues with problem algae in your tank, check out your phosphate levels, to see if that is part of the problem. According to wetwebmedia.com, phosphate levels should be maintained between 0.005 ppm and 0.1 ppm (0.25 ppm is tolerable with hardy corals).


Silicate is a contaminant that can contribute to diatom (brown algae-looking stuff). It is often found in tap water or is introduced into the aquarium via quartz sand.

If you are experiencing a diatom bloom in your tank (brown gunk growing on the sand, rocks, and over your corals), you should test for silicate in your aquarium and your source freshwater to try and determine the source so that you can remove it.

Left uncontrolled, the silicate will act as a fertilizer, supporting the growth of the diatoms, which results in messy, yucky aquarium conditions.