How (and how often) to test aquarium water for healthy fish and plants

Any aquarium hobbyist should know that maintaining good water quality is essential. Regularly testing aquarium water quality should be something we all do when keeping fish, especially in a new aquarium. But do we all know how to test aquarium water correctly? Aquarium water parameters can tell us everything we need to know regarding fish health. Read on to discover how and when to test water effectively.

What does testing aquarium water mean?

Aquarium water testing requires access to a test kit or test strips (at the very least) designed for freshwater aquariums. Fish keepers can use these tools to gain an indication of a variety of different parameters. Ammonia nitrite, general hardness, ph level, carbonate hardness, and nitrate levels, along with a variety of other parameters, should all be within safe levels to ensure the overall health of the fish is not at risk. Aquarium water can quickly become highly toxic to livestock and result in fish loss if left too long. We should do water testing to prevent these accidents from happening.

What about the good bacteria colony in a healthy aquarium?

While a thriving aquarium will have beneficial bacteria, there is only so much these colonies can do to break down harmful compounds. If your fish tank has too many aquarium fish, that creates a lot of waste. Additionally, constant overfeeding results in uneaten food. Both of these examples will eventually cause problems with your water chemistry. Ammonia levels rise, nitrite levels increase, and you end up with cloudy water. Nitrifying bacteria can only do so much, and without regular water changes, your freshwater aquarium will soon cause fish stress, and live plants will begin to show signs of failing. 

How (and How Often) to Test Aquarium Water for Healthy Fish and Plants

How to test aquarium water

For most new fish tank hobbyists, water tests can be pretty overwhelming. Some test kits remind us of our science class at school, and the instructions sometimes take work. We suggest speaking to a team member at your local pet store for help selecting the best kit for you. 

Some people invest in a test kit that can test your water for the leading indicators, leaving other parameters to chance. However, there are test kits available that check absolutely everything. It comes down to how you wish to maintain your aquarium water. Aquarium water testing for many parameters is time-consuming, but if you are keeping a delicate fish species, you will want to ensure the aquarium water is perfect.

An alternative to water testing kits comes in the form of test strips. While not as accurate as test kits, a test strip can give us an indication of things like pH levels, giving test results within a few seconds.

Test aquarium water

What happens if the test shows high levels of a specific water parameter?

You should always check results against a freshwater aquarium water parameters chart when testing your aquarium water. When test results come back high, your first action should be to perform a water change to remove a portion of the water and replaces it with fresh water. 

Many test kits advise correcting the high result by suggesting a chemical with a buffering capacity. However, most aquarium hobbyists will try to avoid adding more chemicals to their aquarium water and attempt to resolve the situation by performing water changes until their tank water testing gives the desired results.

How often should you change aquarium water?


How often should you change aquarium water?

It is generally accepted among aquarium hobbyists that weekly water changes of 10-15% are best for a healthy balance between water chemistry and beneficial bacteria. Occasionally, it is a requirement to remove larger volumes of water, depending on the situation. If, when testing the tank water, you discover high ammonia levels or a ph test colour chart shows a sudden drop, a 30-40% water change is needed.

If a larger-than-normal water change occurs, it's wise to have an additive with a buffering capacity to help your aquarium water return to normal. Both tap water and RO water changes can benefit from this, especially if you live in an area with hard water.

What about cloudy aquarium water?

There are various reasons the water in a newly installed tank can become cloudy. One of the most common causes of cloudy water is related to beneficial or nitrifying bacteria, as it is also known. In a new aquarium, a bacteria bloom can happen before your filter matures or during the cycle period — a typical situation when harmful bacteria is in greater amounts than the nitrifying bacteria. The result is white cloudy aquarium water. If you have already added a colony of nitrifying bacteria, the best course is to do nothing. Performing a water change at this stage would only delay the development of nitrifying bacteria and prolong the process. Once you have a healthy and sizeable colony of nitrifying bacteria, the water will clear itself. 

If the aquarium water has become green and cloudy, this is algae bloom caused by too much light, nutrients or too little maintenance. In these circumstances, a water change will help lower the excess nutrients, but the key to resolving this situation is to eliminate the cause in the first place. Importantly, one strategy often overlooked is the addition of supplemental CO2. In a well-maintained planted tank, CO2 supplementation can improve plant growth, providing an essential carbon source that outpaces the nutritional demands of algae. By bolstering plant vitality and competitiveness, the aquatic environment remains balanced, leaving less opportunity for algae to flourish. Therefore, the strategic introduction of CO2 could play a crucial role in preventing future algae bloom, contributing to a healthier, clearer and more vibrant aquarium ecosystem.

Learn more about cloudy water in the aquarium with this ARTICLE.

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What is new tank syndrome?

All newly set up fish tanks must go through a process to help establish a colony of nitrifying bacteria to create a biological filtration system. This process is called the aquarium nitrogen cycle. The nitrogen cycle is a complex one and involves many steps. First, ammonia breaks down into nitrite, which then turns into more stable forms of Nitrate (a nutrient) that can be taken up by aquarium plants or removed from the water, depending on what you want for your fish.

If we perform a substantial water change, we risk removing too much of the good bacteria colony. This sudden drop in nitrifying bacteria caused an increase in harmful bacteria and will initiate a new nitrogen cycle while the bacteria colony grows to an adequate size. When this happens, we call it the new tank syndrome. 

Sometimes significant water changes are unavoidable, so it is helpful always to have bacteria additives available to reduce the chance of such an event.

What, exactly, should you test your aquarium water for?

As mentioned, some aquarium test kits resemble science labs, offering results on various parameters within your aquarium water. Most freshwater fish have crucial elements that require more attention to help them thrive. Ammonia, pH, nitrite, and nitrate water test kits are integral to aquarium water upkeep. Carbonate hardness and alkalinity tests are helpful, but they only warrant purchasing if you have special needs, such as a planted tank. Phosphate is worth testing for if you have algae problems. Test results should be recorded in a log or journal so that you have a record of what is happening over time.


However, if you decide to maintain your aquarium water, test your water regularly. Buy a test kit that covers the basics or, at the very least, individual test strips and make sure you understand the process beforehand. As a fishkeeper, you must constantly remind yourself that you are also a keeper of water. Look after the water, and the fish will look after themselves.

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