Welcome to the Forum Joe. Glad you came here to learn about your pool chemistry.
Rising pH correlates with an already high(er) alkalinity. Higher pH waters have more alkalinity and that causes the pH to rise more quickly. More acidic, or lower alkalinity waters have less tendency to rise but are less stable and tend to swing more in either direction with regard to pH. Alkalinity represents the amount of buffering capacity a given sample of water contains. More alkalinity equals more buffering capacity.
You can actually raise the pH without raising the alkalinity through aeration such as a waterfall or similar, or a Salt water Chlorine generator. You can also raise alkalinity without raising the pH a great deal by adding baking soda (sodium bicarbonate). The latter is not necessarily something you want to do, and often is not. Your alkalinity is just about perfect at 83 and so is your pH at 7.5. Adding acid lowers pH and alkalinity, and that is a simple fact. Alkalinity and pH are intertwined in a very complex way, and how one effects the other is dependant upon many factors.
As for the amount of Chlorine it took to open, there are some questions that need to be answered. First, how deep is your pool? A 20X40 pool will have a lot of capacity with a very deep end. What shape was the pool in? Was it green or black water upon opening? What else was put in the pool in the past?
Here, we have the philosophy of maintaining our pool chemistry by testing it ourselves with very accurate and dependable testing methods, and understanding what adding any product does. As a rule, we don't trust or follow pool store suggestions because the advice is often wrong, and their test results inaccurate. We encourage others to learn and understand exactly what is going on with their pool and be in complete control through knowledge and understanding. Everything we teach you can learn and most of it is in Pool School. Anything you don't find or understand will be answered by one of the many helpful folks here on the boards. Lastly, and most importantly, this control can't be gained without the use of a quality test kit. Without one, you are flying blind so we strongly encourage you to get one and post results so we can help you properly.
Hang around, read, and enjoy the forum.
