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Fish Tank Water Testing: What to Test For and Why

Fish Tank Water Testing: What to Test For and Why

Posted by Artur Wlazlo on 18 Dec 2020

Ensuring good water quality is one of the most important aspects of maintaining a healthy aquarium. Many hobbyists think learning how to test fish tank water is simple, only to discover it can be the most challenging part of tank maintenance. If you’re trying to learn more about fish tank water testing, read this guide to learn what to test for and why.

The Basics: Water Chemistry

Water chemistry has to do with the chemical properties of the water in your aquarium, including general hardness, ammonia levels, nitrate and nitrite, pH, carbonate hardness, and more. These properties all vary depending on the type of water you use to fill your tank and can be impacted by the fish food you use, the animals you keep, and any treatments you use.

Regardless of the source of your water, it’s good fish tank water testing practice to cycle your filter and biological media to establish beneficial bacteria when you are starting a new tank. This cycle — called a nitrogen cycle — removes or converts harmful elements such as ammonia and nitrites to less toxic ones (e.g. nitrates) that then can be removed with regular water changes and ensures the overall health of your live plants and fish. It can take between two weeks and two months to complete a full cycle.

Chemical Properties

During and after the nitrogen cycle, you should conduct regular fish tank water testing to ensure that all the different chemical properties are within normal ranges.


Ammonia is very dangerous and toxic to fish and can kill them, but it is a natural waste product in all tanks. When you conduct fish tank water testing for ammonia, you never want to see a reading of over 0.0 parts per million. In a fully cycled aquarium, the bacteria present in the aquarium and the filter media will break down and convert the ammonia into less toxic nitrite. Live aquarium plants will also readily absorb any ammonia present in the water column or gravel.

However, there are a number of products designed to remove ammonia from your tank as well in the event your tank is not yet fully cycled or is too heavily stocked, and your testing shows that ammonia is present in your tank.


pH water test monitor

This chemical property measures the acidity of your water. Low or high pH can stress out your fish and jeopardize their health, but frequent and large fluctuations in pH may be even more problematic to your fish. Fish tank water testing pH scales range between 0 and 14. The lower the number, the more acidic your water. The higher the number, the more basic your water. Most fish thrive in water that rates between 6 and 8 on the scale.

Nitrites & Nitrates

Nitrites are generated as the ammonia in your tank is broken down by bacteria. They’re not as harmful to fish as ammonia, but they are still toxic to tropical fish. The only way to ensure nitrites are removed from your tank is to practice proper aquarium husbandry, such as keeping your filters well-maintained and making regular water changes. 

The next step in the breakdown process produces nitrates. This is the final product of the nitrogen cycle and becomes harmful to fish only in high concentrations. To remain safe, your nitrate fish tank water testing kit should show levels between 20 and 40 parts per million. You should perform regular water changes to reduce the amount in your tank. However, live aquarium plants absorb nitrates and are the best natural method of keeping them in check. In fact, in heavily planted tanks, the nitrate levels can drop to zero.

General Hardness

When you’re measuring general hardness, you’re looking at the total dissolved minerals in your water — like magnesium and calcium. A low volume of dissolved minerals is referred to as soft water, and a high volume of dissolved minerals is designated as hard water. Water hardness can affect pH, and the dissolved minerals in moderate amounts are beneficial to both fish and plants. When learning how to test fish tank water, aim for a reading between 5 and 12 degrees (dH).

Carbonate Hardness

A carbonate hardness, or alkalinity, test indicates the stability of your pH. The higher the alkalinity levels, the lower pH fluctuates your tank will experience, resulting in stress-free fish and good live plant growth. If your fish tank water testing kit shows a carbonate hardness between 7 and 12 degrees (dKH), then it indicates a good and stable water pH environment.

Testing Kits at Modern Aquarium

You can find many different aquarium water testing kits in our online store, but a master kit will most likely contain everything you need to examine all the chemical properties of your water. Now that you know how to test fish tank water, you can keep your fish and live plants happy and healthy. Browse our selection of water test kits to check your water consistently!