I have a 6K gallon fiberglass pool. Have had the same water other than rain and top ups for many many years. Never done a full drain. I chlorinate with bleach (Liquidator). I keep all my numbers pretty perfect. TA 110ish, PH 7.4, CYA 40-50, CL 4-6.
Starting about 2 years ago I developed this ALK decrease, PH increase issue. I would say I lose 10PPM ALK per week and PH prob goes from 7.4 to 8.3 each week. Also had a wild scale outbreak earlier this summer that I had to sand off the pool. Water looks perfect right now other than the two creeping chemmies that I mentioned.
Haven't changed my pool filter sand in about 5 yrs. Was planning to do it this fall.
Any ideas? Normal? Does water just get stale after awhile? Considering doing a 3/4 drain and refill this fall.