This could be several things. It's hard to tell from the picture. My first guess would be a bacterial infection and I'd treat with an antibiotic. However this is a case where if I'm wrong, you could do more harm that good.
Cupermine will not alter nitrate readings directly, but it can do this indirectly because it really wipes out your bacteria base. You should also test for ammonia. When your using such treatments, you often need to bring down ammonia and nitrite with massive water changes.