The control of the pH value that indicates the acidity or alkalinity of the water, must be done regularly, because you should know that your fish, as well as the majority, prefer the pH value between 7.0 and 7.5, which would come to be a neutral value, because according to the measurements they are executed on a scale of 0 to 14, with a pH lower than 7.0 considered as acids, and above 7.0 up to 14, they are considered as bases or alkalines.
In order to achieve a stable pH value, the carbonate hardness must always be at least 5° dKH, otherwise there is a danger that the pH value will change (decrease in acids). You should therefore make sure to check the carbonate hardness with the Sera Test kH and, if necessary, increase the carbonate hardness to a minimum of 5º dKH.
Most of the fish that live in freshwater aquariums come from neutral or slightly acidic waters (with a pH value between 6 and 7). Pond fish prefer more or less neutral water (pH value between 7.0 and 7.5). The cichlids of lakes Malawi and Tanganyika prefer slightly alkaline pH values (7.5--8.5).
The ideal value in a saltwater aquarium would be in the range of 8.2 to 8.4.
The pH value should be checked regularly with the sera pH test.