We have a 40,000 gallon plaster pool with a sitting pool that waterfalls into the main. We use liquid chlorine and muriatic acid. The PH and Chlorine is controlled by a CAT 2000 controller. The PH is typically 7.4 and the chlorine runs between 2-3. Pool temperature is 82-84F. Alkalinity was at 40 when first measured. Started adding baking soda at rate of 8 lbs every couple days. Got alkalinity up to 90 by adding about 40 lbs over a couple weeks. Now in the last week or so it has drifted back down to 50 and thick white scale lines are forming on the side of the sitting pool where it waterfalls into the main pool. I am worried scale may also be forming in the heat exchanger as the heater doesn't seem as efficient as it used to be (runs all the time and return water line feels cool). Can anyone help explain what is happening and give me advise as to what to do?