did you ever find the source of zinc?
Maybe?
Years ago, I got in the habit of putting on mild SPF right after showering in the morning. My copper banded butterfly (who came with the new tank) eats out of my fingers, and I was feeding right after getting out of the shower - with those same hands that I had the sunscreen on. Looked at the ingredients, zinc oxide. Then started morning feeding before showering and have done several large water changes since and stuff is looking a lot better. I'll be sending out another water sample for ICP pretty soon.
I also swapped out all jabao pumps so there's only ehiem, red dragon and ecotec's moving water. Pumps were probably not the source (seems everyone runs jabaos with no issues) but I did this before thinking about my fingers in the water every morning.