First post -- hope I'm providing the right info...
I have a 25,000 gallon IG plaster pool (c.1963) with a cartridge filter. Non-permeable safety cover usually on all but one hour a day. I've been using 3" tabs for the past three years and I think they've finally caught up with me. Here are my current test results (I used a Taylor FAS-DPD, then also took a water sample to the pool store, who got identical results):
TA - 130
CH - 350
CYA - 110
No copper present
So two big problems -- high CYA and low pH. For a few years my pH was too high, but in the last two years it's been acidic. Only thing that changed was a switch from Costco tabs (made by Dupont?) to Leslie Tabs, and maybe the cover is on more often? I've tried correcting with Soda Ash a few times, but never really got the pH back where I wanted it (and the pool goes cloudy for a few days). But given my CYA level now, I think it's time to do a more major water change. (I've never changed the water completely in the five years since I bought the house).
Problem is I have small children and never leave the cover off when the pool is unattended. I don't like to drain the pool too much with the cover on as it really sags and puts a lot of pressure on the tracks. The pool store guy said that if you turn the pump off for >8 hours, the CYA will rise to the surface, so that you can then drain from the top (with a pump on the top step) while simultaneously filling from the bottom. I was thinking of getting two flow meters to match the in/outflows on the pump/spigot to keep the water level about right.
Bu then I've read one conflicting post somewhere that said CYA was heavier than water and so dropped to the bottom of the pool...
Has anyone ever tried this? Think this might work?
P.S. I'm going back to liquid chlorine once I get this sorted...