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  • Writer's pictureStewart Kessel

Water Chemistry: pH & Buffering Capacity

Updated: Jan 8

What is pH & buffering capacity - Why are they Important in an Aquarium?

The pH value is a way of recording the degree of acidity or alkalinity of your aquarium water. On a measured scale between 0 and 14, a value below 7 is considered acidic, a value of 7 is neutral, and values above 7 are alkaline (or basic).

In scientific terms, the pH is a measure of the balance of hydrogen ions (H+) and hydroxide ions (OH-). For example, highly acidic water (pH 1.0) has very few hydroxyl ions, but many free hydrogen ions. Highly alkaline water (pH 13) has very few free hydrogen ions but many hydroxyl ions. Neutral water (pH 7.0) has equal proportions of the two. Thus, as free hydrogen ions (H+) increase, the pH is lowered, and the acidity increases.

For those that are scientifically minded, pH is a logarithmic scale, which means for every one-unit of pH change, there is a tenfold change in acid-causing hydrogen ions or alkaline-causing hydroxyl ions. So even a small 0.5 difference in the pH measurement represents a significant change in the environment for the livestock.

Numerous processes within the aquarium environment will affect the pH of the aquarium water. Hydrogen ions are produced as a by-product from the nitrogen cycle as part of biological filtration and will acidify the aquarium water. Carbon dioxide expelled by the fish and other livestock will form carbonic acid and acidify the aquarium water. Detritus and other decomposing waste in the aquarium will release organic acids which acidify the water. Removal of carbonic acid (carbon dioxide) from the water by plants as part of the photosynthesis process will increase the pH of the aquarium water.

It is therefore apparent that many factors influence the pH of the aquarium water, but equally the buffering capacity of the aquarium water also counteracts these effects. The buffering capacity is a measure of how well the water can resist a change in the pH value.

Or to put it another way, Buffering Capacity is the water's ability when adding acids or bases to keep the pH stable

The buffering capacity of the water is achieved primarily through the presence of bicarbonates and carbonates. Hence it is sometimes equated to Carbonate Hardness (KH). However, other bases that can be present in the aquarium water such as hydroxides, silicates and phosphates also contribute to the buffering. These can be strong bases, and therefore although not directly equivalent a better measure of the full buffering capacity is total alkalinity.

Buffering capacity prevents changes in pH by mopping up hydrogen ions, preventing them from causing a decline in pH and hence a rise in the acidity of the water. Equally, when the pH begins to rise too high due to a lack of hydrogen ions in the water, the buffers release hydrogen ions to bring the pH back into balance.

But why is pH Important to Fish and other Livestock?

To understand the effect on the livestock, you broadly have to consider two factors:

1: Habitat: The fish, invertebrates and plants that we care for in our aquariums come from environments where the pH is typically very different from our local tap water and very specific to their local habitat. Maintaining the right pH is therefore essential to help them thrive - exhibiting their natural behaviour and colouration. When kept in water outside of their biological optimum, they can become stressed and more prone to disease.

Many aquarists will keep a variety of fish and other livestock together in a community aquarium. These could have come from different habitats and therefore may not have precisely the same pH requirements but must be selected such that their ideal ranges of water chemistry overlap.

Some fish only thrive in narrow ranges of pH, so this must be a consideration when selecting livestock in comparison to your aquarium water conditions.

Some species of fish live in extreme pH conditions, i.e. either highly alkaline or low acidity environments. Rift Valley Cichlids for example in the wild, are used to a pH value of between 8 and 9. Whilst Cardinal tetras in the Rio Negro region of South America live in water with a pH value below 5.5!

A word of caution is that many tropical fish species have been captive bred for many years in conditions different from their natural environment. Whilst it is valid that their physiology is still genetically coded to thrive in what would be their native conditions, take care when purchasing fish - establish their current water conditions and ensure your aquarium parameters match this. Which brings us nicely onto the next point:

2: Stability of Environment: Beyond specific water parameters such as temperature, hardness and pH, aquatic livestock also need a stable environment. Sudden significant changes or swings in any of the water parameters, including pH, can be harmful and even fatal.

In the case of aquarium water with sufficient buffering capacity, pH stability should not present an issue. Correct maintenance processes will also keep it that way. However, in situations where the buffering minerals have been depleted and not replenished, then dramatic changes in pH are likely. This sudden change will not only cause distress to the fish and other livestock; it will also crash the biological filter cycle, causing further harm to the aquarium inhabitants. For further detail on this scenario, please read our article on Old Tank Syndrome.

Therefore the pH and buffering capacity of your aquarium water are vital considerations when understanding the ideal environment for your livestock and maintaining a healthy aquarium.

Author: Stewart Kessel CChem, MRSC

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