Understanding Your Aquarium Water

Understanding Your Aquarium Water

Testing our water is fundamental to keeping an aquarium or fish tank successfully. By testing our water parameters frequently, we can make minor adjustments to the water chemistry which will keep our fish and/or plant inhabitants happy and healthy. 

Most fish health issues arise from poor water quality and there are many factors which can trigger this including; Age of the aquarium, frequency and quality of maintenance, inadequate filtration, stocking levels and overfeeding

Prevention is better than cure… Therefore by testing our water frequently, we can ensure our water quality is good, and detect any potential issues early before they become a problem. You can do this with an off the shelf, water testing kit such as the NT Labs Aquarium Test Kit or the NT Labs Mini Test Kit.

It doesn’t matter whether you're a novice, or an experienced fish keeper… No one should become so complacent to assume they no longer need to test their water. As luck would have it, most issues tend to occur in the evening when your local fish shop is closed, which makes owning your own test kit empowering. Check your water instantly, identify the cause of the issue and correct the problem efficiently. 

Below we will outline the basics of fish tank water chemistry, which will help you understand what your fish require to thrive. 

Read on to discover more…

What is pH?

pH is the measure of how acidic or alkaline something is. It is measured on a scale of 0 to 14, with 0 being extremely acidic and 14 being extremely alkaline. When looking at the pH of a fish tank or pond, we will normally be looking at a range of pH 6 to pH 8 depending on the species of fish being kept. 

The vast majority of freshwater fish prefer a pH level of neutral (pH 7) to slightly alkaline (pH 8). There are some specialist species such as south american cichlids which prefer mildly acidic water (pH 6.5). Likewise there are certain fish such as Lake Malawi cichlids which require more alkaline conditions (pH 8.5). It is vital you research the fish you want to put in your aquarium and make the pH right for them!

What is KH?

KH stands for Carbonate Hardness. The KH plays an important role in keeping the pH levels stable. If you have a low KH the buffering capacity will be less. This can result in sudden unexpected drops in pH levels which can be fatal to your fish. On the contrary, a high KH means there is a high buffering capacity, and the pH is more stable and unlikely to change. 

The KH levels can be affected by water changes depending on the hardness of your local tap water. For most freshwater aquarium fish, a 2-3 degree KH is ideal.  Regular testing ensures your levels remain within boundaries, and any sudden changes can be detected early. 

While KH is arguably one of the most important elements to test for, it is also the easiest to correct. By adding an aquarium KH buffer, it will begin to immediately increase the KH levels, water hardness and pH.

Please note, not all fish require a KH of 2-3dKH. Some fish like it much harder and others like it much softer. Do your research before committing to a species of fish.

What is GH?

GH or General Harness typically refers to the presence of the general minerals in the water and more specifically, Calcium and Magnesium.

Fish use these minerals in their day-to-day biological functions. In tanks with shrimp, snails, and other invertebrates it is important to regularly test GH levels to prevent mineral depletion. A reading of 3-6dGH is best for most freshwater systems. 

If you find your GH levels are lower than you would like, simply add a GH Buffer to replenish the minerals. Once added it will start to work immediately. 

What is the Nitrogen Cycle?

When setting up a new fish tank, the filter system needs time to mature. Nitrifying bacteria colonies need to grow and develop on the biological filtration media within the filter. These bacterial colonies break down the harmful waste compounds being produced by the tank's inhabitants. This process is known as the Nitrogen Cycle. 

When fish are present in the aquarium, waste and excess food will start to break down producing ammonia. Nitrosomonas bacteria will colonise and grow in the filter system once ammonia is present. The Nitrosomonas bacteria oxidise ammonia into nitrite, while using the energy produced to grow in numbers. Ammonia and Nitrite are both toxic to fish if left to reach high levels so we need bacteria to handle these harmful compounds effectively. A second type of bacteria known as Nitrobacter will begin to colonise the filter media once nitrite is present. This nitrifying bacteria converts the harmful Nitrite to harmless Nitrate which is removed through regular water changes and used by live plants thus completing the cycle.

During the maturation period, the aquarium is extremely unsteady in its water chemistry. For this reason, we recommend initially adding a small number of hardy fish to kickstart the cycle and wait for the ammonia and nitrite read 0, before adding any more. 

The water should be tested at a minimum of every two weeks during this time to better understand how the aquarium is maturing.

Please note, the nitrogen cycle is a perpetual process occuring within the fish tank. It isn’t uncommon to break the cycle thereby resetting the aquarium water chemistry which can lead to issues.


Sudden spikes in Ammonia can be caused by a number of reasons. The most common reason we see is down to overfeeding your fish. When excess food has been left uneaten in the aquarium, it begins to decompose. This produces unnecessary levels of waste which the filter will not be able to process effectively.  

Below we will detail some other common causes, for high ammonia readings: 

  •     Fish may have died and started to decompose.
  •     Plant matter may be breaking down in the tank.
  •     Overcrowding / overstocking your aquarium with fish.
  •     Housing high waste producing fish e.g. goldfish in an aquarium with an unsuitable filter.
  •     Adding too many fish into the tank at one time

Once you identify the cause behind the ammonia spike, it is often easy enough to correct. Firstly you should stop or correct the cause with appropriate action, followed up with a partial water change, alongside a filter clean.  This will dilute the ammonia concentrated in and the water and aid the nitrifying bacteria to break down the harmful compounds. We would always recommend adding in some beneficial bacteria to give the process a helping hand. Fluval Biological Enhancer is very effective and will typically get your levels back to zero within two weeks. Regular testing is always advised to ensure you are heading in the right direction.


As Nitrite is caused by the oxidation of Ammonia, the common causes of Ammonia spikes are typically the same for Nitrite. If you have recently addressed an increase of ammonia, it is not uncommon to see a slight increase in Nitrite immediately after. 

If there are small amounts of Nitrite in your aquarium but no presence of Ammonia, this could be down to a low pH and/or KH. If the pH of the aquarium is kept at overly acidic levels for a period of time, the bacteria in the filter can begin to die making the conversion rate less and less effective.

Once you have stabilised the pH levels, you can add in some Seachem Prime to the aquarium. This will help to detoxify any residual Ammonia and Nitrite while building up the bacterial reserves in the filter system.


Nitrates are the end-product of the Nitrogen Cycle. Most freshwater fish are very tolerant of Nitrate, even at high levels. However, just because they can tolerate them, doesn’t mean you should have a high Nitrate in your aquarium.  Precautionary measures can be taken to prevent too high a build-up of Nitrates in the aquarium.

You can add in live plants to your aquarium, they will feed and thrive off the Nitrates that are being produced. The more heavily planted your tank, the lower your Nitrate levels should be. In an unplanted tank manual removal via water partial water changes is a crucial process. By removing around 10% of the water on a weekly basis you are helping to reduce nitrates and maintain a stable ecosystem. Remember to always use Tap-Safe when topping the tank back up with tap water to ensure it is dechlorinated and safe for your fish!

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Looking after a fish tank can be challenging if you don’t understand basic water chemistry. Having a basic understanding of pH, GH, KH and the nitrogen cycle will improve our chances of success in the hobby. 

Having a stable, established aquarium will prove to be the most cost effective and rewarding practice. For any advice related to the contents of this article, please don;t hesitate to contact us directly. Click here to get in touch today. 

Shop the best aquarium products & accessories for your fish at Complete Koi today

For more information and advice on aquatics, check out the rest of our blogs…

How Much Electricity Does My Fish Tank Use? | How To Safely Clean And Maintain An Aquarium |A Beginners Guide To Setting Up An Aquarium 

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