If you are purchasing fish from a fish store, ask the attendant about the pH value of the water in which the fish were kept. This would be ideal aquarium water pH and should be maintained in the aquarium at your home or office to ensure the safety of fish.
From the same fish store, purchase a pH test kit. The kit should cost you somewhere around $10. With any given test kit, the number of tests that can be conducted vary from 30 to 75 tests. Instant Ocean test kit, Marine pH & Alkalinity Multi Test Kit and High pH & Alkalinity Test Kit are commonly used pH test kits.
You will have all the more reason the check aquarium water pH if there is a sick or deceased fish in the aquarium. Even if everything is alright, you should make it a practice to check pH of the aquarium water at least once every month
Frequent or abrupt changes in the pH value can be directly related to the water’s KH or carbonate hardness. Just like a pH test kit, a KH test kit can be purchased from a local fish store. You will have to use a buffing compound if problems with the pH are being caused due to fluctuating KH or carbonate hardness.
Make sure that you thoroughly read the instructions detailed in the instruction's manual for both the pH and KH kits. Consider making a bar chart to keep record of the aquarium water pH. Analysing those charts will prove to be a positive step towards safety of the fish while they are in the aquarium.