Have had this pool since 2008. No particular problems. Just found out something interesting.
My CYA was at about 150 at the tail end of last season, and thereabouts after running the pump a bit for opening(getting new reagents now). (Never-to-be-sufficiently-accursed Trichlor pucks before I learned better). Did not drain pool down for the winter (lack of time - I'm on the road a lot).
When I opened my pool, I did my usual routine, which includes dumping in a bottle of "Frog Bam", a copper based algecide (after initial water test). Have not added any metal sequestration products since maybe year #1. Did NOT add much water (maybe 2" - I use a skimmer faceplate plug and bullseye plug for winter). This is a 15' round ABG, 54" wall (6000 gallon). Hayward 3/4 HP power flo matrix. Hayward EC40 DE filter. Clear Solar Cover. Vinyl liner. Del Ozone Big Dipper Ozone generator. Nothing else special. Chemistry not too bad, but I'm going to have to work on my TA (High).
After the SLAM (with appropriate compensation for CYA level), and PH adjust, just to knock down the grungies, the water turned pretty much crystal clear. (2 Days). But I had a considerable amount of purple stuff in a ring around the bottom of the liner.
Did a little investigation and blew the dust off my college chemistry books. I believe it is copper cyanurate. Took a considerable amount (understatement - A LOT)of scrubbing to get it off. Tested water afterward. Now CYA is at 75 (again, my reagents are old. Taylor TF-100). Seems to be borne out by some aquacheck strips I had. And the calculated SLAM chlorination sent my FC WAY over where it was predicted to go, tending to bear out that a good bit of the CYA was eaten up by the copper in the algecide.
Anybody seen this before?
And NO, NO, NO. I'm NOT suggesting doing this on purpose. The beautiful cyan/ amethyst colored salt created is a PAIN to get off the vinyl. Would probably permanently stain plaster or fiberglass.