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Why should you test your aquarium water?

Date Published - 22nd November 2012 - Published by - iQuatics Ltd

Testing your aquarium water is a recommended part of fishkeeping, yet it is vastly overlooked by a lot of hobbyists. What should be tested and how often is a common question and one that there is no short answer to. In a newly set up aquarium water testing is vital to know when it is safe to add livestock; a new aquarium needs to go through the nitrogen cycle before livestock is added (also known as the fishless cycle). During this process toxic levels of ammonia and nitrite will build up before being converted into the less toxic nitrate. You can read more on the nitrogen cycle here.  In established aquaria water testing is important to safeguard the health of your livestock.

Test kits should be considered as a staple part of the aquarium set up and operating expense of your aquarium and if you do not feel comfortable testing your own water or can’t afford the outlay of a test kit you should speak to your local fish shop before setting up to find out what their policy is on testing customer’s water. Some will offer free water testing others will charge a small fee for the process. In the long run it is usually more cost effective to invest in your own test kits.

What test kits should I get?

There are many different brands and types of test kit available to today’s hobbyist, from test strips which are an all in one dip test right through to digital testing devices. All have their positives and negatives. The best advice I can offer is to do some research, find out what others think of different test kits and most importantly choose the kit that suits your needs and pocket the best. They can still give you an indication that something may be wrong, it’s better to test with something than to not test at all!
Ammonia (NH3), pH, Phosphate (Po4), Nitrate (No3) and Nitrite (No2) are the basic essential tests that should be carried out on every aquarium. There are many other tests that are available such as General Hardness (dGH), Carbonate Hardness (dKH), Magnesium (Mg) & Calcium (Ca).


Let’s take a look in more detail into each test and why it is performed.


This test is essential, especially during the initial set up of your aquarium. Ammonia can be found in more established tanks if the water is not changed regularly, any filters are not kept clean, if the tank is overstocked or any medication is used that disrupts the biological cycle of the tank. It is recommended in a new tank that ammonia is tested weekly until it is no longer detectable, in a mature aquarium this test can be done monthly. However if at any point you get a fish death, you should test for ammonia immediately, it is tremendously toxic to fish.

pH can be the most frequent cause of fish stress, which in the end can result in the loss of a fish. Unfortunately it is normally the most overlooked test. Most fish cannot stand sudden changes in pH. Even a small change can result in stress of your livestock. pH can and will change over time, fish and other waste, evaporation, top ups and water hardness will all contribute to changes in pH. A rule of thumb is to test pH once a month or if there is a fish death or illness. pH can be stabilised using a pH buffer but this should be done according to manufacturer’s instructions and done slowly.

During the initial set up stages of a new tank nitrite levels will be very high which is why it is important to cycle your tank using the fishless cycle as nitrite is toxic to aquatic life. Even when your aquarium becomes established it is not un-common for them to go through what’s known as a mini cycle from time to time. For that reason it is recommended that nitrite is tested for on a monthly basis; any sign of increased nitrite levels can be the first sign that there may be a problem arising.


Although less toxic than nitrite or ammonia nitrate levels should be monitored to avoid adding unnecessary stress to your fish. Nitrates can also be the source of nuisance algae problems. The closer you can keep your Nitrate levels to 0 the better.


Whenever anyone grumbles that they cannot win the battle against algae, phosphates immediately come to mind. Phosphates act as a nutrient for algae and raised levels will certainly add to your algae miseries. Although it’s rarely mentioned, a leading cause of increased phosphates is dry fish food – particularly overfeeding with lower quality foods that are high in phosphates. If you have algae overgrowth, test for phosphates. There are filtering materials available that remove phosphates.
So we have covered the basic essential tests that every aquarist should be performing on a regular basis. Now we can take a brief look at other tests that may be required in more advanced aquaria.

General Hardness

The General Hardness (GH) of your aquarium water is a measurement of dissolved magnesium and calcium. The (GH) of aquarium water can have effects on your tropical fish so it is very important to ensure that the tropical fish you choose to keep will thrive in your water.

Carbonate Hardness

The second element of aquarium water hardness is the Carbonate Hardness (KH). (KH) is the measurement of carbonate and bicarbonate ions in your aquarium water. In simple terms, the (KH) of aquarium water is a measurement of the buffering capability of your aquarium water. The (KH) of aquarium water will determine how much your pH will fluctuate. The higher your (KH) is in your aquarium water the more stable your pH will remain. When (KH) drops, so too will your pH. There is a distinct relationship between (KH) and aquarium pH. So if you keep having problems with your pH dropping, look to see what your (KH) levels are in the tank water.


Quite a straight forward test, this tests the amount of magnesium in the water. To give a proper reflection on why people test for magnesium would require a long scientific answer, but for easiness sake, think of magnesium as calcium wanna-be. During calcification, occasionally magnesium can be chemically substituted for calcium. This means higher magnesium levels can allow more calcification (a.k.a. growth) to occur. There are side effects for extreme levels (too low, or too high).


Calcium itself, is used by the animals in the tank mainly to expand their skeletons (i.e. grow) such as hard corals and inverts. Calcium is also inversely tied into alkalinity. If calcium levels drop, alkalinity will normally rise and vice versa. Balance of the two is the goal.
People who have been keeping aquarium for many years will profess to having “an eye” for things. This only comes from experience and getting to know your aquarium. There are also two other tests that are vitally important, one we should all check and the other that should be monitored by those that keep marine aquaria.
The first is temperature, ensuring a stable temperature is vitally important and something we should all be monitoring on a daily basis, it only takes a second to check thermometers and heaters are all working as they should. The second that is focused toward the marine hobbyist is salinity, again it is vital that salinity is maintained in the reef aquarium.
So we have now covered the basic tests and a few of the more detailed tests that people perform on their aquaria. As mentioned in the opening paragraphs there are no hard and fast rules for water testing and quality. Each person will have a different method and different ideas as regards to water testing. The best advice we can offer is that only you will know how often you should be checking things, you know you aquarium better than anyone else.
You have just read another great aquarium blog post by iQuatics. If you would like us to blog about a specific subject or have your own aquarium blog content you would like published on our website, please get in touch. Together we can help grow the iQuatics aquarium blog into a vast resource full of combined industry knowledge.

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