High combined chlorine

[chem geek]

If they were acidic Trichlor pucks or granular Trichlor (which is uncommon), then that can degrade plaster where it is in contact or nearby since Trichlor is acidic, but it is the acidity that is the problem, not the chlorine. For other sources of chlorine or higher chlorine levels, they won't normally be a problem for plaster. With hypochlorite sources of chlorine, one may also be adding acid and THAT can be a problem for plaster if poured in one place away from a return flow since it can pool at the bottom if not mixed.

[Adolfsmith69]

My sister in law has been maintaining a large pool for a year now with a copper ion system. She has not had to add any type of chlorine which suprised her and me. She installed two systems because of size of pool. Copper rods last about 6 months x 2 sets at $250 each plus. She also added a ph monitor and pump for around $2500. Pool still needs constant monitoring to keep copper level right.

[FreeBee]

I used Oxyshock from my pool store and that just killed the combined chlorine.
The ingredients are listed as potassium monopersulfate 30%
It was a little expensive but cleared the problem in a couple of hours.