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How to Test the Water in Your Fish Tank

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When thinking about water quality, it’s important to remember that the very thing that your fish lives in is water. It is all around them, they ‘breathe’ it through their gills, so the quality of your fish tank's water is critical to your fish’s health, wellbeing and ultimately their survival.

You should test your aquarium frequently to keep an eye on any changes in the makeup of the water. You should also check the water quality if your fish are looking sick or you have had a death in your tank.

What do I need to test for?

We recommend that you test your fish tank regularly, fully testing on these main four areas; Nitrite, Nitrate, Ammonia and PH. When you first get a fish tank we recommend that you test the water once a week to get your head around what is happening with your water and how it changes. Is there anything you are doing that is causing these changes? If you do have a high level of something what you do to bring these levels down?

Nitrite aquarium testing

When you are first starting up a tank the nitrite levels will most likely increase as the tank is establishing the biological filtration. High Nitrite Levels are a concern as high nitrite levels can stress or worse yet kill your fish.

If your nitrite increases in any way it is a concern that the biological filtration is not being as effective as you thought, so a water change is the best place to start to decrease the nitrites.

Nitrates aquarium testing

Although nitrates are not as toxic as ammonia or nitrites, they must be monitored to avoid stressing the fish. Nitrates can also be a source of algae problems.

If you have any level of nitrates we suggest a 20% water change, it is best to try and get on top of them as soon as you can.

Ammonia aquarium testing

Testing for ammonia is one of the most critical water tests. Ammonia is a build-up of any waste, left over fish food or plant matter in the tank. If the biological filtration is not working well then there will be a rapid build-up of ammonia. We suggest you test for this weekly as you are unable to see ammonia build up. The only sign might be the fish gasping at the top of the water.

Once a tank matures ammonia can still build up if regular maintenance is not performed, with regular water changes and filter cleaning. Regular maintenance helps to keep ammonia build up down.

If you experience any ammonia in your tank, we suggest a 20% water change as soon as possible and re-test the water within 24 hours. If you are still having trouble with ammonia then we recommend a further water change, make sure that your filter is cleaned out and gravel siphon to remove as much matter as you can from the tank that may be causing the build-up.

PH aquarium testing

PH can also be a frequent stressor and cause of fish death. It is the sudden fluctuations in PH that cause the most damage to fish.

It’s important to know the PH of your tank at home and also the PH of the water that is coming into the tank from the tap. You are best to let Tap water sit overnight before testing its PH as the water will have dissolved gasses as a result of being under pressure. Make sure that any water you put into the tank has the same PH as the water in your tank.

Also if you are adding fish from a shop what is their PH, will there be a sudden change when you put the fish into your tank?

PH does change in a tank over time, so it is also important to test this weekly to begin with so you are well aware of what is happening with your tanks water. Once you are confident the PH is stable then you will be able to test on a monthly basis.

If your water PH changes suddenly, or changes over time, you should check the KH (Carbonate Hardness) of the water. A PH test will help you see if you have a good buffer in place that helps to maintain the stability of your tank.


Phosphate is a food for algae, so if you are having algae problems test your phosphate levels to make sure this is not the problem. Phosphates are easy to remove.

Types of aquarium test kits available

Master aquarium test kits – this kit enables you to conduct all of the main tests so is a good starting point to get you going.

Individual aquarium test kits – these are great to refill your master test kit, or if you have a specific problem that you want to test for.

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Written by The Team

A team of specialists with backgrounds in animal nursing, animal care, and all things pet related.