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Cleaning Your Aquarium Part 4: Regular Water Testing.

Aquarium water testing

The regular and consistent water analysis of your aquarium is the most powerful tool for a healthy and beautiful aquarium. Nothing is more effective at forming an accurate picture of your aquarium’s health than accurately and precisely testing your water parameters.

Water analysis is only effective if done correctly. To get the most out of water analysis you must:

  • Test the same parameters consistently

  • Test regularly

  • Chase trends, not numbers

  • Use accurate and precise testing equipment and techniques

The Importance of Testing Aquarium Water Regularly and Consistently.

One of the pillars of the scientific method is consistency. In terms of aquarium water analysis, this means testing the same parameters at the same intervals, on a regular basis. In order to identify trends and patterns, we need multiple data points over a consistent and regular period of time.

It is not enough to simply perform the occasional nitrate, phosphate, or pH test. These tests are only useful if you have a data set, taken over time, to compare it to.

For example, if you're trying to determine the rate of nitrate build up, you might test nitrates before every water change. Or maybe you test nitrates once a month. Whichever frequency, or regularity, you choose, being consistent and testing at the same time, every time, is key.

Being consistent to the time of day can be important as well. Some parameters, like pH, can fluctuate up and down throughout the day. If you test in the morning one day, and in the evening on another, you will see a misrepresentation of the actual daily fluctuation.

Aquarium Water Analysis Reveals Trends

You’ve probably heard to not chase numbers when it comes to aquarium water analysis. Chasing numbers means picking an “ideal”, sometimes arbitrary, metric for a certain parameter and then constantly trying to hit that target “ideal” metric by altering the chemistry of your aquarium with additives, filtration, water changes, etc.

Trying to chase a specific number can result in more harm than good, and oftentimes is an impossible feat. A more effective method is to keep your aquarium parameters within a safe range and only take action when you see your parameters trending outside of that range.

Trending can be defined as a parameter moving in one direction over at least three different tests, or data points. One or two tests do not reveal a trend, you need at least three tests to determine a pattern or tend. More data is usually better if that data is accurate.

For example, there is no difference in the health of your aquarium between a calcium reading of 420 and 400. That difference could also be a testing error. However, if you test at 420, then 400, then 380, that could suggest your calcium is trending down and action should be taken to correct it.

Which Aquarium Water Test Kits are the Best?

Alkalinity Tester
Colorimetric Alkalinity Tester

Aquarium water parameter test kits can be divided up into hobbyist, professional, and lab grade. Hobbyist grade test kits have a wider range of error and rely on methods like colorimetry. Professional test kits are more affordable versions of lab grade tests. They use many of the same methods, but without the scientific grade precision. Lab grade test kits are much more accurate and precise. They use methods like ICP-OES, titration, and spectrophotometry.

Saltwater Titration Test
Titration Test Kit

There is nothing wrong with hobbyist grade test kits. Just be sure to know the range of error when using these kits. For example, Hanna’s calcium test kit has a standard error of plus or minus 15 ppm. If you test calcium twice and the readings are different, but within 15 ppm, your calcium concentration may not have actually changed. This is one of the reasons we chase trends, not numbers.

Lab-Grade Aquarium Testing

If your budget allows for professional or lab grade test kits, they can be a powerful tool. Some of these kits are more trouble than the sometimes minor increase in accuracy or precision is worth. In these cases, it may be better to choose a less accurate kit that will be consistently used rather than a more accurate kit that just sits on the shelf.

Lab grade testing can also be done in actual labs through several companies via the mail. Utilize these resources as often as you like, but at least a couple times a year. This way, you’ll never miss that build up or depletion of an element you can’t test for at home.

What Aquarium Water Parameters Should I Test For?

There are many parameters which should be kept track of to maintain a healthy aquarium. Not every parameter needs to be tested at the same time. In general, the faster a parameter is capable of changing, such as salinity, alkalinity, or pH, the more often it should be tested. Parameters like nitrate and phosphate can be tested less often.

Below is a short description of the most common water parameters to test for.


As one of the most variable parameters, temperature should be constantly monitored. Using a thermometer, adhesive thermal-meter, or temperature probe attached to a monitor, the temperature of your tank should always be readily available and easy to read.

The air conditioner being turned up on a hot day, a window being left open, or a busted heater can quickly change the temperature of your aquarium. Having a constant readout prevents dangerous fluctuations.


In saltwater aquariums, salinity can change quickly over the course of just a few days. Salinity can be affected by neglecting freshwater top offs or dosing alkalinity and calcium. You should test the salinity of the aquarium before every water change to determine the salinity to mix the new water to. New water should be tested before being added to the aquarium as well.

Freshwater aquariums should not have measurable salinity levels. Sometimes, freshwater aquariums can benefit from the addition of some sodium chloride. In these cases, the part per thousand should still read below one.


Saltwater Aquarium Hydrometer

Testing salinity can be done with a hydrometer, refractometer, or digital tester. Floating, glass-stick hydrometers are the simplest, most accurate, precise, and never need to be calibrated. Microbubbles can alter the readings of the plastic-lever hydrometers. Handheld refractometers can be precise and accurate, but only if you test multiple times and use perfect technique. Temperature, light angle, and sample volume can affect a handheld refractometer's readings. They also need to be calibrated before each use. Benchtop refractometers are generally more precise than their handheld counterparts. My personal favorite is a digital conductivity tester. These devices are accurate and precise, easy to read, and only need to be calibrated every few weeks depending on frequency of use.

Digital Salinity Tester Saltwater Aquarium

Regardless of which is your favorite, it is best practice to measure salinity with multiple methods to be sure your readings are accurate. My favorite combination is a glass,-stick hydrometer and a digital tester.


In both freshwater and saltwater aquariums, pH can fluctuate throughout the day. As CO2 levels rise and then fall, its relationship with alkalinity causes pH to fall at night and rise during the day.

Saltwater Aquarium pH

pH fluctuations are important to monitor in saltwater reef aquariums. You need to know what your lowest point and highest point during the 24 hour cycle is. The smaller the fluctuation from 8.3, the better. For example, 7.8 at night and 8.0 during the day is considered a wide fluctuation and is too low from the ideal 8.3. Whereas 8.1 at night and 8.3 during the day, is the same fluctuation, but is much closer to the ideal number of 8.3.

Testing however many times is necessary to determine how much your reef tank fluctuates is important. Consider starting with a pH test each morning, afternoon, and evening. For fish-only saltwater tanks, just test before each water change to see if your pH is drastically off from diminished alkalinity.

Freshwater Aquariums

The pH in freshwater aquariums should be tested before each water change to determine if it has changed since the last water change. Freshwater planted tanks that use CO2 injection should have pH monitored weekly and exactly 24 hours after CO2 levels are adjusted.


Liquid reagent test kits for pH testing are the most common and easy to use. However, they can be inaccurate if the exact, correct amount of reagent isn’t used. Test strips are quick and easy, but can be difficult to read and have a low degree of accuracy.

pH Checker Salterwater Aquarium
pH Checker

A digital tester that uses liquid reagents can suffer from the same pitfalls as any other liquid reagent test kit. My favorite method for testing pH is a handheld digital probe. While they need to be calibrated fairly often, depending on usage, they are more accurate, precise, and trustworthy.

Single junction probes are great for freshwater and double junction probes are perfect for saltwater.

Like with any test, I recommend using two different types of testing kits or equipment when determining the initial accuracy of your equipment, especially if you are using kits or equipment that can’t be calibrated with a standard solution. After you're confident the method you have chosen is accurate and precise, then limit the confirmation testing to approximately once a month.


Much like with pH, alkalinity also fluctuates throughout the day. In reef aquariums, alkalinity is used up by corals during the day and dips at night. In freshwater, alkalinity remains fairly constant, but can fluctuate slightly when animals, including the microbiome, respire and release CO2.

Saltwater Aquariums

Alkalinity can be measured in reef aquariums as often as multiple times a day, or only once before every water change. The goal is to minimize fluctuations in alkalinity as much as possible. If you measure weekly, you will be able to correct alkalinity after it's been dipping for a week. This could be quite a large dip. If you measure every hour, you can correct on a much more precise level and the alkalinity in the tank will never measurably dip.

The best practice is somewhere in between these two extremes. If you have very little livestock in the aquarium, start by measuring alkalinity before each water change. As you add more corals, your alkalinity will begin to dip more than 1 point in a week. When this happens, it's time to start dosing an alkalinity buffer in between water changes to keep your alkalinity stable.

Be sure to test your alkalinity before each dose if you're dosing manually. If you are dosing automatically, begin by testing consistently and regularly. Start by testing alkalinity (1) when your lights come on, (2) half way through your light cycle, (3) when your lights go off, and (4) half way through your night cycle (or as close to it as you can get).

These readings will tell you how much your alkalinity changes throughout a 24 hour period. Use this data to adjust how much and when the buffer is dosed in order to shrink the fluctuations as much as possible (preferably within 2/10ths of a degree).

Once you have a week of consistent data, and you're confident your alkalinity isn't fluctuating too much in a 24 hour period, then you can pull back on testing to between once daily and once weekly.

Freshwater Aquariums

Alkalinity only needs to be measured before each water change in freshwater systems where the alkalinity is stable. This reading will inform you of how much alkalinity buffer to add to the new water. If heavy feeding leads to high rates of decomposition and therefore large bacteria populations, testing alkalinity in between water changes may be necessary to determine if dosing is necessary.


Neptune Trident Water Analyzer
Water Analyzer

For freshwater systems, a basic titration kit is sufficient. For reef aquariums, I would suggest a liquid colorimeter test kit, or a quality titration test kit. There are also automatic water analyzers available in the reef hobby that will automatically test, via titration, alkalinity, calcium, and magnesium on a set schedule, multiple times a day. If one of these testers is in your budget and you are automatically dosing any of these testable parameters, I recommend getting one.

Calcium & Magnesium


Known as general hardness (GH) in the freshwater aquarium, calcium and magnesium are used by plants and animals at a relatively low rate. Best practice is to test for these elements via a titration GH test kit before each water change to ensure new water has been treated appropriately.


In the reef aquarium, calcium and magnesium are highly important. Depending on how many corals you have, and whether or not you are dosing, it's a good idea to test for these elements, individually, at least once a week, or daily if you have many corals using lots of calcium and magnesium.

Follow the same testing guidelines for alkalinity when testing calcium and magnesium. Like with alkalinity, corals will uptake less calcium and magnesium at night. The amount and timing of your dosages are important to monitor. Testing multiple times a day, at the same times initially, will give you an accurate and precise dosing schedule.

Calcium and magnesium fluctuations do not affect coral growth as much as fluctuations in alkalinity. While these parameters should remain relatively stable, they do not need to be dialed in to the degree that alkalinity does.


Freshwater GH Test KIt Aquarium

For freshwater, the basic 1-part liquid reagent test kits for general hardness (calcium and magnesium) are sufficient. For saltwater reef aquariums, I recommend either a liquid/powder colorimeter or a titration test kit for each element separately. Like with alkalinity, there are automatic water analyzers available that will take multiple readings per day.

Ammonia & Nitrite

Ammonia and nitrite are most associated with the nitrogen cycle. Most people don’t bother testing for these compounds after their tank is cycled. This is mostly true for nitrite. As the second stage in the nitrogen cycle, it is the shortest lived and is rarely measurable after the nitrifying microbiome is established.

Ammonia on the other hand is a direct byproduct of fish waste, fish respiration, excess food, and even underperforming RODI filters. Some systems even have limited nitrifying bacteria and every time you feed, a measurable amount of ammonia is present. For these reasons, it's a good idea to test your ammonia/ammonium levels every month. If you have ammonia present, it's a good idea to look at how much habitat for nitrifying bacteria you have, if you're feeding too much, or if your RODI filter is letting ammonia or chloramines (which can read as ammonia) through the membranes and resins.


In most cases, a 2-part liquid reagent test kit will suffice for ammonia and ammonium measurements. However, whenever I detect considerable ammonia or ammonium levels when I wasn’t expecting them, I always confirm with a higher quality liquid reagent kit that separately tests for total and free ammonia.

Nitrate & Phosphate

These two molecules are some of the most debated pieces of chemistry in both the fresh and saltwater hobby. Regardless of where you land on how much of each should be measurable, the principle behind their production, accumulation, and measurement is the same.

Nitrate and phosphate are considered inorganic when they are free-floating in the water column. This is when they are measurable by our testing kits. Nitrate and phosphate are organic when they are attached to biological organisms (inside bacteria) or as part of other organic material (fish waste).

Nitrogen (especially in the ammonia and ammonium form) and phosphorus are more biologically useful in that they are more easily absorbed by animals and plants than nitrate and phosphate. We can estimate the amount of nitrogen and phosphorus in the aquarium by measuring the nitrate and phosphate.

The exact desired concentration of nitrate of phosphate is different for every hobbyist and every aquarium. However, measuring these nutrients at least once a month will prevent build up beyond undesired levels and allow you to notice trends, whether good or bad. Personally, I test for nitrate and phosphates at least once every two weeks, at most once a week, or once a month if I've been showing stable readings.


For general readings with a desired accuracy less than 10 ppm for nitrate, I recommend a 2-part liquid test kit. For all phosphate readings, and nitrate readings with a desired accuracy less than 1 ppm, I recommend a powdered reagent colorimetric test kit, such as the Hanna checkers.

Potassium, Iron and Other Trace Elements


In the freshwater aquarium, potassium and Iron are important elements for plant growth. If you have fast growing plants, getting a potassium and iron reading every month or two is not a bad idea. If you're not fertilizing a heavily planted or fast growing planted tank, you're definitely low on both of these elements. If you are dosing fertilizers, knowing how much to dose is important.

Other trace elements in the freshwater aquarium are ideally replenished when new water is treated with a remineralizer, an all-in-one fertilizer, or trace fertilizer; and therefore don’t need to be tested. These include elements like molybdenum, zinc, nickel and others.

Honestly, I would test for these elements if I could. However, a reliable, inexpensive test kit for these elements suspended in a liquid solution is not readily available. In lieu of testing, adding trace elements as directed, then adjusting based on visual analysis of deficiencies is best practice.


In the saltwater and reef aquariums, all these minor and trace elements are in the salt mix and are replenished to a degree with every water change. If you have growing corals and are dosing alkalinity and calcium, you're probably using trace elements at a rate faster than they can be replenished with a water change. If this is the case, it's a good idea to test for these micro and trace elements right before a water change using an “ICP-OES” lab-grade test. These should be done approximately every three to six months to check if you're deficient in minor or trace elements. They are also useful in determining if you have contaminants in the aquarium.

Liquid reagent test kits for potassium and iron are available for freshwater and saltwater aquariums. Test kits for strontium are also readily available for saltwater.


One of the most crucial elements of a saltwater reef or freshwater planted tank, light rarely gets measured the way it should. Knowing the intensity, spread, and quality of your photons will allow you to accurately and safely place corals and plants while avoiding too much or too little light in the tank; leading to bleaching or excessive algae growth.

PAR Analysis Aquarium
PAR Meter

Accurately testing your light’s photosynthetically active radiation (PAR) can be costly; the tools to do it right can cost over $600. However, if you know someone with a submersible, lab-grade PAR meter, or a way to rent one, it is well worth it to measure your PAR and adjust your lights as necessary. If done correctly, you should only have to make this measurement once as you will rarely have to move or change your lights unless the aquarium or the aquascape changes.

This may be an expensive measurement to make, but you should only have to make it once. Be sure to record your PAR readings as you make them across your tank. When you add new plants or coral, you will know the best place to put them per their requirements.

Photosynthetically Useful Radiation (PUR) is just as important, if not more so, than PAR. PUR is essentially the intensity of the individual spectrums of light, not the intensity of all the spectrums together (PAR). PUR measures the quality of your light, what is actually photosynthetically useful vs just the quantity of photons (PAR).

Corals, for example, love bluer light near the 420 nm range, while plants prefer a more intense red part of the spectrum. Unfortunately, there is no hobbyist-grade, readily available equipment as of yet that can test for PUR. The best practice is to research your animals and plants, know the spectrums they need, and adjust your light’s individual spectrum intensities to match that research.


In freshwater high-tech planted aquariums, CO2 gas is injected directly into the aquarium. The rate at which this gas is injected is different for all aquariums. No matter the setup however, the parts per million (ppm) should be between 20 and 30. Once you achieve this concentration, you shouldn't have to re-measure CO2 again unless you adjust the CO2, or just want to confirm your injection rate every once in a while.

There are two ways to measure CO2. The first is to take a KH measurement and a pH measurement then use this table to find your CO2 concentration. The only issue with this method is you can get false readings if there are any other factors besides pH or alkalinity (KH) that are affecting the CO2 concentration. For example, some shrimp salts and pH buffers will give you a high, false CO2 reading because they lower the pH without lowering the KH.

The second way is to use a two part liquid test kit and measure the CO2 concentration directly. This method is the more reliable of the two, but requires another test kit, although they are fairly inexpensive.

Visual Testing

Testing kits and equipment are a vital tool to understanding our aquariums. They can also help us diagnose and track problems and solutions we may be experiencing.

The easiest, and sometimes most reliable test we can perform, is a visual assessment. It is crucial to set aside all the kits and equipment and visually observe the aquarium regularly and consistently.

Take additional time to deliberately observe problems you've been having, and the growth or recession, health, behavior, and color of your livestock. Note the algae growth, the flow, and amount of detritus build up. Go beyond just looking at your aquarium as a normal part of enjoying it. Make intentional observations, note them down, and relate them to the maintenance you have been performing.


Every aquarium and aquarium hobbyist is different. There are a thousand ways to test your aquarium’s parameters correctly. Different regimens, schedules, and testing equipment work better for different hobbyists. In the beginning, don’t worry as much about being perfect, just be accurate, precise, and consistent.

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