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The Importance of Water Chemistry

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 a very mature aquarium, 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?

For any of you considering your water chemistry, there are several things to look out for.  First off is ammonia, your fish produce this as waste, just like we do.  Your fish do not have access to a separate toilet though, so the ammonia they produce simply mixes up in the water.  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 on your test results 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 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.  Again, 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.

In a new aquarium, these will build up pretty quickly if it is not cycled properly, which is why I always recommend fish-less cycling.

Nitrate is the least toxic of the three, but ideally, you need to keep this figure below 10 ppm.  The level at which it actually becomes dangerous is around 40 ppm, lethal at 80 ppm.  So whilst 10 sounds pretty low in comparison, it is still poison and if it was me in there I think I would like it to be zero where possible.  Nitrate is less likely to be broken down by your filter as it requires a bacteria that works anaerobically (without oxygen).  As our aim is to provide oxygen for our fish, it is in the water, so this bacteria cannot survive in the aquarium.

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.  Plants also help as they consume it.  The most effective means of reducing these levels though is water changes, and this is why we do them.  Even the most mature of aquariums require regular water changes to reduce nitrates.

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

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 loss.  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.

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 can be fixed by adding buffering compounds to your water to keep increase the alkalinity.  These are usually available in most aquarium shops.



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, which is available in most aquarium shops

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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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