This is just an educated guess, but I'll bet your Total Alkalinity (TA) level is sky high. Also, do you have any aeration features (waterfalls, spillovers, fountains, etc.)? Test your water with a Taylor K-2006 test kit or equivalent (Leslie's Complete, for example) and tell us your Free Chlorine (FC), Combined Chlorine (CC), pH, Total Alkalinity (TA), Calcium Hardness (CH), and Cyanuric Acid (CYA) levels.

Richard