Thanks for any insight you all can give me on this puzzle. I've had this pool for 11 years, and this is the biggest water problem so far.
This winter I didn't lower the water, just left the solar cover on and ran the pump when temperatures got below freezing. I planned to keep some Cl in the pool, but neglected it, and ended up with extensive algae, including enough biofilm to eat up all my CYA.
Two weeks ago I raised CL to 16, removed as many leaves and debris as possible (couldn't see the bottom), added a bit of CYA, and made a halfhearted effort to clean the bottom of the solar cover. As per recommendations here, I've continued to add Cl to maintain shock levels and have run the filter 24x7. Still can't see the 8.5' bottom after 14 days. While there is some improvement - I can now see my brush when its about 7 feet deep, while a few days ago I couldn't see it at 4 feet deep - this cleaning process is far and away the most protracted I've encountered. I've never had to go more than 4-5 days previously to get sparkling water.
Test results from last night, which are typical:
Borax between 30 and 50
CYA - not registering, should be about 10 given what I've added so far.
22,000 in ground pool, sand filter, vinyl liner.
I have been removing the solar cover at night and replacing it during the day to keep Cl levels high.
After the initial 24 hours the pressure in the filter wasn't going up to levels requiring a backwash. Nevertheless, I was backwashing approximately every 48 hours. The last few days I've added some DE to the filter, thinking there must be stuff in the water not getting filtered out by the sand. This has led the pressure to go up more quickly, and I've ended up backwashing about daily, but the cleaning process still seems awfully slow.
What I haven't done:
1) Throw away the solar cover. Perhaps I should, as the algae certainly had made a nice home for itself there. However, after 2 weeks the bottom of the cover looks pretty clean.
2) Vacuum to waste. I know that can speed up such clean-ups. The last five years regular brushing has kept the pool clean, so I didn't replace my cracked hose.
The persistent CC readings of 1.0 to 2.0 indicate there's more work for the Cl to do. So what do you suggest? Just stay the course and let the Cl work? Or try something else?