I have never does copper into my tank any tips on how to do it?Impossible to treat without knowing what the issue is.
You indicate your losing fish, this can be ick.
You can try Ruby Rally, but I stick to copper and hypo, they will work. We don’t know what’s in some in-tank treatments.
If fish keep slowly dying, one by one but not fast, Then I might jump to the conclusion, ick is present.
who’s left?