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What to Test in Your Aquarium: The Essential Guide to Water Parameters

If you’ve ever watched The Princess Diaries, you may remember when Lilly says to princess Mia, “Are you really sure you can run a country? You can barely keep your goldfish alive for more than a couple of days!”

Some of us may know this sad experience all too well, or know someone who does. While I don’t know what killed Mia’s goldfish, I have a strong suspicion that she failed to pay attention to the water parameters when she brought it home with her.

What many beginners fail to realize is just how important water testing is especially with a new aquarium and a new fish. It’s easy to think we will get around to it after we’ve settled into our rhythm, but knowing the “what” and “why” behind testing parameters early on will save you a lot of trouble in those early days.

Let’s take a look at the most important water parameters to test for, and then compare the different ways this can be done.

Guide to Test Kits

The reason these four parameters come first on our list is because dangerous levels can inflict the most immediate negative effects on the fish. The other benefit to prioritizing these is that the results can also send up red flags about the other parameters that we will discuss, prompting us to know what else to test.

Both freshwater and saltwater aquariums should be tested for these four parameters on a weekly basis. When starting a new aquarium, however, the beneficial bacterial colonies need time to establish themselves, so you will want to test every 2-3 days for about three weeks. If levels appear stable, then you can slow down to only once per week.

Potential of Hydrogen (pH)

The pH test is a measure of hydrogen ions in the water which give it either acidic or alkaline properties. 

The pH scale is measured from 1 to 14, with 1 being the most acidic, 14 the most basic (alkaline), and 7 being neutral. Most freshwater fish thrive best in water that is maintained between 6.5 and 7.5 pH.  Saltwater fish can do better in a slightly more alkaline environment, ranging usually between 7.8 and 8.5.

Sudden changes in pH can cause stress, or even be fatal, to a fish. When you purchase your fish, ask a store associate what the pH of the fish’ aquarium is. You should also do your own research to determine what the ideal pH is for this particular fish. Then test the aquarium water which you plan to use before adding the fish. It is generally best to gradually add aquarium water to the water of your new fish before fully releasing it into the aquarium, so as to avoid shock.

See our article Balancing pH Levels for more information on this topic.


The chemical compound NH3, otherwise known as ammonia, is a gas that can be present in the water especially in the early days of an aquarium’s startup. Once the water has had a few weeks to actively build up the needed bacteria, the nitrogen cycle is more effective as the ammonia is broken down into nitrites and eventually nitrates. You cannot see ammonia, but you can smell it at very high levels. It is produced by fish waste and food waste. It is especially caused if there is a death of a fish in the water, in which case you should immediately test the water for elevated levels.

Regular cleaning, water changes, and a good filter are essential to keeping ammonia levels at the targeted zero parts per million (ppm). Even with slight elevations from zero, this can cause stress to your fish and weaken their immune system. If it rises above 0.25 ppm, action needs to be taken immediately to reduce back to zero.


One type of bacteria will break the ammonia down into nitrites (NO2), which is also toxic to fish. Fortunately, these nitrites are quickly broken further down by the second type of bacteria into nitrates, far less toxic. Nitrites, like ammonia, should be kept at zero ppm. Even slightly elevated levels can cause stress to fish, potentially causing rapid fatalities if it reaches 5 ppm or higher.

As with ammonia, nitrite levels have a strong likelihood of being elevated in the first several days after an aquarium’s setup. It could continue even for a few weeks while the bacteria become established, so testing every few days will be very important.


Here we can take a deep breath and relax: nitrates (NO3) are not great for fish, but they’re nowhere near as toxic as the preceding ammonia and nitrites. It’s important to include these in a test simply to make sure they aren’t getting out of hand. Anything below 50 ppm is generally considered safe, and even below 100 is not likely to have serious effects in the short-term. You still want to target 50 or below but have time to act. Some fish are more sensitive to this chemical compound than others, so it is prudent to research and make sure you know the ideal parameters for your specific fish.

Great news: plants love nitrates! In fact, they absorb nitrates for use in photosynthesis, which means they remove nitrates and replace them with oxygen in the water. Talk about a symbiotic relationship with your fish!

3 More Parameters to Monitor

While the previous four measurements are of utmost importance for the early days of your aquarium, the next three tests are just as important for their long-term effects on your aquatic environment.


Alkalinity is important for the long-term stability of pH. It is a measure of the water’s ability to resist changes in pH. In other words, it acts as a buffer.

Measuring alkalinity is specifically examining the presence of alkaline compounds such as carbonates, bicarbonates and hydroxides.

To read more about alkalinity, check out our article How to Raise or Lower Alkalinity in the Aquarium.

Carbonate Hardness (KH)

For aquariums, the alkalinity and carbonate hardness (KH) are almost used interchangeably to mean the same thing. The reason for this is that KH only measures the presence of two compounds: carbonates and bicarbonates. While it leaves out other compounds that are generally included in the measure of alkalinity, it’s important to note that carbonates and bicarbonates are generally the most prevalent compounds in water which contribute toward alkalinity. Others, like hydroxide, play a more minor role. Except in rare circumstances, if you measure KH, you can draw a reasonable conclusion about the overall alkalinity.

In saltwater aquariums, the typical target range is 7-11 dKH, whereas a freshwater aquarium is closer to 3-8 dKH. 

General Hardness (GH)

A measure of minerals in the water, such as calcium and magnesium, is important to ensure that your fish are exposed to the nutrients they need for biological processes such as metabolism and bone growth. These nutrients can be absorbed over time, but can also be easily replenished through the use of specific supplements or by including such things as limestone, seashells, or crushed coral in your substrate or filter.

Signs of general hardness that is too high (hard water) can be seen in the calcium deposits that may build up on the sides of the aquarium or on the decorations.

Most freshwater aquariums target a GH level of 4-8 dGH. Saltwater aquariums generally test individually for calcium and magnesium levels.

Honorable Mentions

There are a number of tests that can be done on the water, so here are just a few less popular tests which you may consider if you suspect an imbalance.

  • Phosphates – You may want to test for these if you see algae growth.
  • Chlorine – Tap water often includes some chlorine. High levels can harm fish externally and internally.
  • Carbon Dioxide – Too much carbon dioxide can harm fish. If they appear to be gasping for breath, either with their mouth at the surface or by rapid gill movement, this may be a sign that CO2 levels are too high. Plants can help with maintaining a balance.
  • Copper – Small quantities of copper in the aquarium can be beneficial by eliminating parasites, but if levels go too high it can be toxic to fish.

Saltwater aquariums benefit from additional tests not mentioned here. There is a lot of overlap, though. The most popular tests for saltwater include:

  • Alkalinity
  • Salinity
  • Temperature
  • Nitrates
  • Phosphates
  • pH
  • Calcium
  • Magnesium

For more information about the suggested parameters for different types of aquariums, check out Hanna Instruments’ article.

Types of Aquarium Test Kits: Pros and Cons

Now that we have an idea of what we’re testing, what is the practical application of this in real life? Let’s take a look at the specific ways the water parameters can be tested.

Test Strip

Among the most popular ways of testing is the test strip. This consists of submerging a long strip into the aquarium water which then causes certain colors to appear on the strip. You can then compare the shades of color with a color guide in order to determine approximately what the specified levels are. This is popular for its ease of use and its speed; the whole test takes about one minute to be completed. A major drawback, however, is that it only gives a very broad idea of the levels and has been known to give false readings at times. If you are going to use this method, consider using two test strips from different manufacturers to cross reference and compare.

Liquid Test Kit

This type of test seems to generally be favored slightly over the test strip in terms of accuracy. It simply involves taking a small water sample in a test tube and adding drops of a solution which will turn the water a certain shade of color to be compared with a color chart. This will need to be done for each parameter that you are testing. It is slightly more time consuming, but not significantly so.

Digital tests

For pH levels specifically, you can use a digital tester which very simply requires you to dip the reader into the water solution and wait for a few seconds to allow the digital display to adjust and present the reading. This has been celebrated for accuracy and speed. It is limited to reading only pH, as well as water conductivity, which is related to, but not exactly, the hardness of the water (GH).

Continuous Alert Readings

If there is one or two particular parameters that you’ve had difficulty maintaining, you may want to try this unique method of sticking a display on the inside wall that continuously reports a reading of that parameter. There is a margin of error which you’ll have to account for, but it is a great way to remain on alert for any dangerous levels of the measured parameter. These typically last anywhere from 6-12 months before needing replacement.


For the more advanced saltwater aquarist, you also have the option to send samples to a lab for testing via ICP-OES. This stands for “Inductively Coupled Plasma – Optical Emission Spectroscopy.” In this way, you can test for a very broad range of different elements, even beyond what over-the-counter tests can offer. Knowing these parameters can be important to sea life, and especially to maintaining a healthy environment for corals.


I like to think of water parameters in relation to monitoring the flight instruments on an airplane. Instead of flying a plane, though, you’re running a miniature ecosystem. It’s an easy thing to overlook, or to do only on occasion, but making a habit of regular testing and adjustment will work wonders to the health and longevity of your aquatic life. Now you know the basic building blocks of water parameters as well as the different way in which to monitor them. Maintaining this balance will make all the difference between failure and success in this hobby. The long-term effects of tending to these invisible elements may well make you the top aquarist among your friends!

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