My swimming pool gives mild electrical shock with all power off. I have a copper ionization purification system. What should I look for? The electrician does not know pools or copper ionization.

How mild? Above ground or in ground? Kill the breaker to the filter and the pump, still getting mild shock? In the meantime stay away from the pool and kill the breaker 24/7 until we figure this out.

It sounds like the wiring is wrong inside the subpanel. Test for 240 or 110 at the timeclocks or pump if you do not have a timeclock. If you get a reading of 240/110 at the pump/timeclock with the breaker off then it is a breaker. Electrolysis in the water would not even be noticeable. If the problem does not lie with the subpanel then there could possibly have been a underground powerline/cable that is damaged. Either way I highly recommend that you contact a pool guy and have them recommend a good electrian that knows about pools to you.