It depend from fish to fish IMO. Lets say arowana have stronger immune system than discus fish. If they are in the same pond, discus fish might get affected by the pathogen and arowana might not show any symptom. Poor water quality might stress some fish and weakens its immune system, allowing invaders to enter its skin. IMO its better to separate the sick fish and give treatment accordingly with regular WC (aged, aerated, and same temperature water). Make sure water is good quality and give antibiotic (melafix) to heal up the wound. Once fish is healthy introduced it back to the pond. Its just my opinion, hope it helps.