Please help! Planted discus tank.Water parameters fine. Established tank. One fish refused food two weeks ago. Turned black, white poop. Water changes carried out every day, temp raised, treated with wormer. Fish begins to eat when a small mark appears on head, clamped fins, hiding and turns black again. Treated with hexamanita. Fish starts eating last night, much lighter in colour. I come in from work to a huge white mark on head, fish black. Any suggestions? I don't know what else to try. Rest of tank is incredible healthy and active. Thanks Jen