Different species of fish around the world evolve naturally in water of differing, but relatively stable, pH values and, although most species can tolerate small changes of pH over a few days, large changes will cause stress and, in severe cases, fatality.
pH is measured on a scale ranging from 1 (extremely acidic) to 14 (extremely alkaline) with 7 being the neutral value. Fish which require an acidic water prefer a pH value around 6.5 and those requiring an alkaline water prefer a pH value of around 7.5.
Although this may seem like a small range of values, it must be noted that the pH scale is not linear so a pH change of only one equals a tenfold change in acidity or alkalinity.
The pH value of your aquarium water can easily be checked using one of the many water
test kits available. They involve taking a small sample of water and either dipping
a small strip of chemically impregnated paper or dissolving a small tablet in it.
In the first instance, the colour of the paper strip will change colour and, in the
second case, the colour of the water will change. In both cases, the colour is
matched with a set of test colours provided with the kit.
Once the pH value of the water is determined, its value may be changed by using a pH adjustment kit to suit the types of fish you wish to keep. However, for most amateur fishkeepers, it is generally more practical to determine the pH of the local domestic water supply (which will be used for periodic water changes) and only keep species of fish which are suited. Although the pH of the domestic supply can vary over a small range, it does tend to remain either alkaline or acidic.
A high pH in the aquarium may be due to excessive plant growth or rocks such as limestone or coral sand. Remove some of the plants and replace the rocks with either artificial ones or use slate, sandstone or granite.