When considering a new aquarium, the hardness and pH of your tap water is the first thing you should evaluate, once you know the characteristics of your water supply it will enable you to choose fishes that would be suited to your water conditions. Alternatively you will have to adjust the chemistry of your tap water to suit the requirements of the fishes that you choose.



The term pH refers to the power of hydrogen, and is measured on a logarithmic scale of 0 to 14. As you may have noticed pH is closely linked to water hardness, generally speaking, a higher concentration of salts means a higher pH, and a lower concentration of salts means a lower pH.

A pH of 0.0, the strongest acid, represents a solution normal in hydrogen ions (H+).

A pH of 7.0, being neither acid nor alkaline and considered neutral and having equal numbers of hydroxyl and hydrogen ions.

A pH of 14.0, strongest alkaline, represents a solution normal in hydroxyl ions (OH-)

You would need a thorough knowledge of basic chemistry to understand the true meaning of pH in water, and I'm no expert, so for our purpose we will define acidity and alkalinity in terms of the number of ions found in the water.

If there is an excess of hydrogen ions (H+), then water is said to be acid, if on the other hand there is an excess of the negatively charged hydroxyl ions (OH-), then the water will be alkaline.

Being logarithmic means that a change from pH 7.0 for instance, to pH 6.0 is actually a ten-fold increase in acidity, or a ten-fold increase in hydrogen ion (H+) concentration. To give a more powerful example:

pH 8.0 is 100 times more alkaline than pH 6.0 or a 100-fold increase in the negatively charged hydroxyl ion (OH-).

It is necessary to maintain a stable pH, and in the correct range, to keep tropical fishes healthy and colourful.

As a general guide, egg-laying fishes such as:

Tetras, Angelfish, Rasboras prefer a pH of 6.5,

While the live bearing species such as:

Mollies, Swordtails, and Guppies thrive at pH 7.5.

When keeping a mixed community of tolerant species in the aquarium you could have the pH at a neutral pH of 7.0

Excessively acid or alkaline conditions in the aquarium, as well as pH fluctuations should be avoided, it will cause stress to your fishes, and a stressful environment leads to lower resistance to disease, poor fish colour and poor appetite. Any changes made to the pH of aquarium water should be conducted slowly over a number of days to avoid stressing your fishes.

A minimum pH of 6.5 is sensible; filters are less efficient below this due to the reduced growth and survival of nitrifying bacteria. Species requiring a pH below 6.5 should have frequent water changes to prevent the accumulation of nitrogenous waste.

As mentioned earlier, pH is closely linked to water hardness, if your pH is high then it is probable that your water is high in calcium bicarbonate, which is an effective pH buffer, and acts against processes which try to change pH.

For this reason I use a commercial pH adjuster, which sets the pH at a fixed level. It then holds or buffers the pH at that level and fights against pH rebound (ordinary adjusters only temporarily change pH, and water can very quickly return to its previous condition, this is known as pH rebound). Unfortunately, there are a number of naturally occurring processes in the aquarium that can threaten to alter the perfect pH that you have provided for your fishes.

The filters task of breaking down toxic waste ends in the formation of nitrates, which are a salt of nitric acid and can exhaust the pH buffer and cause sudden drops in pH. This can be prevented with regular water changes (which are a part of regular maintenance anyway), which will dilute the nitrates; this will be more effective when coupled with a thorough cleaning of any accumulation of debris in the substrate.

Carbon dioxide (CO2) is introduced into the water by the respiration of fishes, amongst other things, when this dissolves in the water it forms carbonic acid (C2CO3), this too can add to a drop in pH. Good aeration and circulation assist in the removal of CO2 into the atmosphere, and reduces its effect on pH.

The surface area of an aquarium should be wider than it is deep; this gives a greater surface area for any given volume of water, which aids the gassing off effect of unwanted elements, and is also greatly beneficial for the absorption process. The two elements, hydrogen and oxygen, are chemically bonded together and are not readily freed. Therefore, fish cannot use the oxygen that is in the water molecules.

Water is capable of dissolving many substances, and oxygen is no exception. Oxygen is constantly being absorbed from the air, by the water's surface and diffuses through it. Fishes utilise the dissolved oxygen from the water by means of gills; then excrete their carbon dioxide waste back out through their gills into the water. In this way gills are comparable to lungs.

Aerating your water with either air stones run from an air pump, or via the venturi of an internal power filter or power head will help in the process of oxygenation, the longer the bubbles take to rise to the surface the better the chance is of oxygen being absorbed from them. However, that is only part of the process; the slowly rising bubbles are also able to carry waste gases, such as carbon dioxide expelled from your fishes, to the surface and into the atmosphere, which in turn reduces its effect on pH.