Yup - that's what I meant by structure...just places for the fish to hide and feel secure. Whether live or not, that is important. And that structure is "alive" in the sense that it is hard surface - places where bacteria, important in biological filtration, can grow.
txjigsaw -
Well indeed, copper would be a reasonable assumption when you observe those symptoms. Quite classic. Have you had a test for copper? What is your water source? Have you ever added copper or medication to the tank?