First, here in the Phoenix Area we have pretty “mineral filled” water, so we need to change out the old water and add fresh water about every 3 years. During the year, the high calcium content in our water mixes with dirt, sunscreen, body oils, etc.. This causes a layer or film on the interior of the pool. Although it might not be obvious to the naked eye, almost every pool we have acid washed has had this layer. If not addressed, it can shorten the life of your pool finish and cause serious staining that cannot be removed.