There are several factors that can determine the make-up of the water in your aquarium and understanding some of these can help you to maintain your tank in peak condition.


This is the most common area hobbyists have issues with.  pH is a measurement of the acidity or alkalinity of the aquarium water. 7 is neutral, above this up to 14 is alkaline and below this down to 0 is acidic.

All fish species have different preferences and ranges in which they thrive.  In the wild, lakes are often alkaline due to leaching of limestone that lines the base. Rivers such as the Amazon are acidic due to leaching from soils and clay. This excess of wood and mud in areas like South America will lower the pH. Drier places with clay grounds and billabongs such as Australia also support a slightly acidic pH.

Most fish species will adapt to a range of pH values. It is, however, important to try to maintain a stable pH value. Over time most species of fish can adjust to small changes. Cichlids are an example of a fish with that require a higher pH. Limestone and coral based rocks and substrates are placed in the water to create a more alkaline environment with a pH of 8 or higher. Most South American Tetras and Australian rainbows will prefer a lower pH of around 6.5.

To change the pH you must change the water buffer level. The best way to bring the pH down is to use a commercial plant substrate, add wood to the water and peat to the filter. Peat and wood are great but may initially stain the water with tannins which may yellow the colouration of your water. Powdered commercial buffers also work but they are not an exact science. They will need to be added to each water change to ensure the new water being added is the same pH as that in the aquarium.

Raising the pH for housing species such as Cichlids is fairly simple. Substrates such as crushed coral and Ocean Rock offer a white substrate and chemically buffer the water to a pH of 8+. Limestone chips can be added to the filter to increase hardness and also boost pH.

Water Hardness

Water hardness can be most simply described as the mineral levels in the water. Hard water has a high dissolved mineral content. Soft water has very little. The most common mineral in water is calcium; however, other minerals may also be present. Most people’s tap water is either slightly hard or soft depending on where it comes from. Well water from areas which have a lot of limestone (calcium) is often hard. Water that comes from lakes and rainwater is often devoid of minerals, making it soft.

It is important to understand how water hardness affects pH in your aquarium. Hard water (high mineral content) is usually high in pH. Soft water (low mineral) is usually low in pH. The mineral in hard water acts as a buffer in the water. The resulting water will be more alkaline and higher in pH. Some species of fish require hard water while others require soft.

So what do we do? Well it is not too hard to add mineral in the form of calcium based rock, so making soft water hard and more alkaline (higher in pH) should not be too difficult. To soften hard water, you need to take the mineral out with a water softener, reverse osmosis, or a specialized chemical that irreversibly binds up the mineral. Another option is to find a source of de-mineralized water for your fish tank.

Of course the alternative to all of this may be to tailor your fish and plant species around your existing water source. For beginning aquarists this may be the best solution. There are a wide variety tropical fish available and it is not difficult to find at least a dozen different species for every different type of water. Any decent book on aquariums and tropical fish will list the individual pH and hardness requirements of the different fish species.

We measure the hardness of the water in your aquarium in General Hardness (GH) and Calcium hardness (KH).

Ammonia (NH3/NH4+)

We test for the presence of Ammonia in the water as it is highly toxic to your fish. All fish waste and uneaten food turns to ammonia and ammonium. Whilst ammonium is far less toxic than ammonia, we can only test for the combined amounts. The ratio between ammonia and ammonium is dependent upon the pH level with a lower pH having more ammonium and a higher pH having more ammonia. We should be aiming for zero ammonia in a water test which shows that the nitrification cycle is working well by converting the ammonia into Nitrite.

Nitrite (NO2)

The bacterium that accumulates in your filtration breaks down the ammonia and converts it into Nitrite. This also is highly toxic to your fish and we should always be aiming for a zero reading in your water tests.

Nitrate (NO3)

This is the end result of the nitrification process, and although relatively harmless to your fish in small doses we should be aiming for between 20-40 parts per million (PPM) in your tests. Weekly water changes with fresh de-chlorinated water removes a portion of the nitrates in your water. For example, if your Nitrate level is 40 PPM and you replace 25% of your aquarium water you will be removing 25% of the nitrate (40 PPM will become 30 PPM) so it is best to stick to a rigid regime of water changes regularly rather than sporadically, as more water will need to be changed to lower nitrates which have been allowed to accumulate.


Although not exactly a water parameter, keeping your aquarium’s water at the correct level for your fish is important. Fish are cold blooded and rely on the surrounding water temperature. Try to keep the temperature stable and not fluctuating. Higher temperatures decrease the amount of oxygen available in the water.


Often overlooked, phosphates can build up in the substrate and is the end result of decaying and dissolved organic matter. Ideally, phosphates should be kept to a minimum by regular deep cleaning of the sand/gravel with a vacuum substrate cleaner. When phosphates build up the result may be unwanted algae and bacteria blooms which may be detrimental to your fish and plants. Using activated carbon within your filtration can help to reduce phosphates in your aquarium. Remember to remove carbon if medications are used!