The use of copper

[sky]

I did a huge water change, took all the rocks out and put them into a freshwater bin soaking with copper above 4.0 ppm (mg/L) for almost two months after a terrible ich (or oodinium may be, I can't tell) spread in my tank which killed almost all my fishes. Putting the rocks back and slowly adding some new fishes in but disappointed to see there is still some ich (a few on their fins and tails) on both the survivors and the new comers. Even though they all eat fine and the ich seems like under control/not spreading widely (it's the 3rd week now from adding the new fishes), but I'm worried that the threat will happen again when I add more fishes later on. In order to stop the misery from happening, should I add copper directly into my tank now? or wait and see like I did before? Will copper break down the biological cycle happening in my canister filter? And is it wise to leave copper in my tank permanently and will fish become weaken for being soaked in a safe copper concentration level at 0.3 ppm (mg/L)? Since some of them don't have ich and will they become weaken after treated by copper? Can shrimps and crabs stand copper?

Btw, I have a 90 gallon running w/ a rena xp4 and coralife hang on skimmer

Thank you for your time

Sky