The pH values should be between 6.5 and 8.5. (Typically tap water is about 7, dependant on the area of the country). Test the pH of the water using ‘pH test strips’. You will need to locate the source of the high/low values though.
If the pH is high >8.5 then this could be high nutrients or calcium leaking from concrete in or around the pond. If there is any untreated concrete, it can be sealed. Nutrients can be used reduced by carrying out a series of partial water changes.
If the pH is low <6.5 then the possible causes are rotting vegetation, too much fish food being given (only enough to be eaten in 5 mins), inefficient filtration and overstocking.