Thank you PoolDoc for your reply and suggestions!

Yes, you are right, my PH level was way too low over winter (possibly also in the fall) and I was putting chlorine tablets in the skimmer. I fixed PH and alkalinity now and I am not putting tables into skimmer anymore. I will bypass the heater and hopefully reduced the problem in the future.

You are also right that I have DE filter which I just replaced (Pentair 59023400 Complete Element Grid Assembly Replacement 48 Square Feet FNS Plus Pool and Spa D.E. Filter), because the old filter could not do the job anymore.

I am do not know if copper is only sequestered temporarily, or if maybe part of copper is indeed removed. Water testing shows lower level of copper, which could be due to both, I guess.

Is ia very bad to add so large quantities of sequestrant (12lb of super Sequa-Sol up to know and still adding)? Should I rather replace water?

Thanks!