It is important to create the correct amount of buffering in the pool such that the pH can be adjusted if necessary but rapid fluctuations are prevented. Low alkalinity can result in corrosion and discomfort to bathers. Free chlorine plus combined chlorine . This apparent contradiction in terms refers to conductive chemicals that can accumulate in the pool particularly when the water evaporates, or when the pool is not 'diluted' with sufficient fresh water. They are mostly made up of chlorides and sulphates.
Chlorides can accumulate with long term use of sodium hypochlorite.
Another stabilised chlorine donor. It is called trichlor because there are three atoms of chlorine bonded to the nitrogens on the molecule. This makes it stronger than dichlor which only has two. A mildly scale forming situation is preferred as a thin coating of calcium will protect the metal fittings in the circulation system. There is a formula to test whether a pool has water which is corrosive or scale forming called the Langelier Water Balance Formula.