What control the pH of the water?

It depends on the softness or hardness, soft water e.g Amazon river is acidic so below pH 7 and is caused by things like decaying vegetation and the elements present in the surroundings. Hard water e.g african lakes are alkali so above pH 7, this is again caused by surrounding areas and elements such as calcium which make it alkali. In the aquarium it can be controlled by chemicals which you can buy at pet shops.