My hot tub is about 18 years old and the gel coat is not perfect. Whenever I clean the tub and refill it, after about 3 days, any copper that was in the water, collects on the imperfections in the tub as black marks. The chemicals are right and if I notice the copper reading while the tub is still clean, it is fairly high. Then all of the copper comes out of the water (low reading on water test) and binds to any crack or imperfection in the gel coat. I have tried SeaClear to give the metals something to bind to and get filtered out but it doesn't work. Any suggestions?

Bromine.
Vancouver, BC Water.