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The Importance of Water Chemistry – Aquarium water testing

Water Chemistry

Water Testing – Checking Water Chemistry

Water chemistry is a key factor in keeping a healthy aquarium. I’ve seen many new starters looking for help online with aquarium issues over the years, only for them to then state that they don’t own a water testing set.  If you’re new to aquariums then I can tell you that a testing kit is essential, never more so than at the start.

Even as an experienced aquarist with very mature aquariums, I still like to have a test kit handy for the occasional check.  If for any reason you are unable to acquire a testing kit, any good aquarium shops will be happy to test your water for you, sometimes for a small fee, but if you’re lucky for free.


So what are we looking for in aquarium water testing?

For anyone starting a new tank, the cycling process will be your first experience of water chemistry. Whilst your aquarium cycles there are several things to look out for.



First off, your fish produce this as waste, just like we do.

I think we can all agree that if we were in our fish’ place, the tank would become pretty uncomfortable the moment we went to the toilet.

Fish waste produces ammonia, which is highly toxic, so as fish keepers we rely on bacteria in our filters to break it down into less harmful substances.

Once you have a mature filtration system this ammonia will be dealt with too quickly for your test to detect it, and that is one of the ways that you can be sure that your aquarium is cycled.

Ammonia is toxic even in low levels so if your test shows any ammonia above 0.25 parts per millilitre (ppm) and you have fish in your tank, you will need to conduct regular (daily/twice daily) water changes until your filtration can convert the waste effectively (regular tests required).  The only safe level of ammonia is zero.



Next up in your testing kit should be nitrite, this is what ammonia is converted into after it is consumed by bacteria in your filtration system.

Nitrite is also quite toxic, but not quite as toxic as ammonia.  Nitrite can cause stress to your fish at levels as low as 0.5 ppm and is lethal at 10 ppm and above. Once your aquarium is cycled, nitrite should be undetectable.

Like the ammonia, it will be consumed by bacteria quickly enough that your test shouldn’t detect it.  After consuming the nitrite as food, this bacteria expels nitrate.



Nitrate will build up in most aquariums and the best way to remove it is by changing the water. This is one of the reasons we do weekly water changes.

Nitrate is the least toxic of the three chemicals we have mentioned so far, but ideally, you need to keep this figure below 10 ppm.

The level at which it actually becomes dangerous is quite high for most adult fish. It is best to keep nitrate below 80ppm to keep fish healthy, but as low as possible is best.

Nitrate is less likely to be broken down by your filter as it requires a bacteria that works anaerobically (without oxygen).

One of our aims as aquarists is to provide oxygen for our fish, which we purposefully ensure is in the water, so this bacteria cannot survive in the aquarium or in the filter.

Nitrates cannot be eliminated from the aquarium but they can be controlled.  Some filter media is quite porous and will allow small colonies of anaerobic bacteria to survive which can help.

Live Plants also help as they consume it as a nutrient.

The most effective means of reducing these levels though is water changes and this one reason why we do them.

Even the most mature of aquariums require regular water changes to reduce nitrates and to replenish its mineral content.


If you are new to aquariums and you are looking for more details on cycling then check out the ‘Seriously Fishy Book’, it’s free on Amazon’s Kindle unlimited.




The importance of pH and KH

The pH of aquarium water is regularly overlooked when thinking about water chemistry but is just as important as any other test.

Even small sudden changes in pH can cause fish to go into shock and potentially die.

The pH of your water can be affected by various things, fish waste, plant waste, water hardness, so it needs to be tested regularly.

It’s also worth checking the water from your tap fairly frequently too in case it changes.  This is the case even in an established, mature aquarium where it should be checked at least once a month ideally.


KH or Alkalinity

The KH (carbonate hardness) or alkalinity of your water can affect your water pH.
Over time the carbonates in your water can be used up which causes it to lose its buffering ability meaning it cannot hold a stable pH.

This is often referred to as ‘old tank syndrome’. Where your KH has depleted meaning your water can no longer buffer (keep stable) the pH of your water.

Maintaining KH is another reason for regular water changes. Adding freshwater adds more carbonates, helping to maintain your tanks buffering ability.



Phosphates are not particularly dangerous, but if you’re having trouble with algae then it’s worth testing for.  Phosphate acts as a nutrient for algae so it may be what is causing your tank to be full of it.  Phosphate is usually caused by lower quality fish foods that contain higher levels of it.  There are filter media available that will help to remove phosphate, but if you are having trouble with it persistently then it’d be worth shopping around for some new food for your fishy friends.


Avoid the loss

If it hasn’t become clear already, water testing is really important for aquarists of all experience levels.  Small changes in your water chemistry can often mean the end for your fish if a change occurs suddenly or continues for an extended period of time.  So always invest in a water testing kit.  Whilst some of them seem a little expensive, it’s far better (in my opinion) to shell out a little cash than to end up with deceased fish and be unable to tell why.


If you’re keen to check your water chemistry and carry out water testing, below is a link to a pretty comprehensive review of aquarium testing kits if you wish to read more. However, in my experience, you can’t go too far wrong with the API master test kit, or the NT labs kit which are available in most aquarium shops, or you can grab them here:


API Master test kit

NT Labs Master test kit


(As an Amazon Associate I earn on qualifying purchases).



Test kit review:

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I am a fish keeping enthusiast with over 20 years experience. I currently keep American Cichlids (CA) which are my favourite fish to keep so far. I started Seriously Fishy as I noticed a large volume of people on various web pages looking for help with Aquarium basics. I created the first Seriously Fishy book to solve the issue in 28 pages, that led to the Seriously Fishy UK fish forum and blog.

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