Bicolors are more susceptible to copper than any other dwarf angel for some unknown reason.
In the old days, prior to reliable test kits, and as unfortunate as this sounds, we used bicolors as the proverbial "canary" for dosing copper into tanks.
They always were the first to suffer at lowish doses. This was copper sulfate, and I am aware Cupramine is a milder form of copper.
All I'm saying is dose with care.