The pH is in all respects a measure of acidity and alkalinity, pH 0 - 6.99 is acid; pH 7.0 is regarded as neutral and pH 7.01 - 14.0 is alkaline. On the whole the pH is not generally a problem but it can have a profound effect on the toxicity of ammonia. Alkaline water, that is with a pH of over 7.01 in combination with increasing temperatures causes more of the ammonia to exist in the free form, which is very poisonous to fish.
The higher the pH and water temperature the greater percentage of the ammonia which exists in this harmful free form. If the water is alkaline it is worth bearing in mind that this will affect the toxicity of ammonia and that even very low readings could therefore be quite serious for the welfare of the pond fish.
The pH of the pond is largely dependent on the pH of the make-up water in the surrounding area and therefore it is not possible to try to control this parameter artificially.