what to treat with

[Baltimore Bryan]

Recently, my damsel has been acting strange- rapid breathing, slightly clamped fins, darkened skin around the gills, scratching on gravel, etc. The only other fish in that tank is a six line wrasse, which acted fine. I treated entire tank with General Cure (by API; it contains 250 mg metronidazole and 75 mg praziquantel) medicine, which definitely helped but the damsel isn't fully normal, he is acting/ breathing fine but he still has darkened gills. I think its external parasites since his poop is dark brown not white and stringy. I was going to treat with Mardel CopperSafe but it's instructions are not clear and I've heard it can kill fish. Any suggestions on what I can treat that will effectively cure them? (It doesn't have to be reef safe since this is a fish only tank)
Thanks,
Bryan

 

[Gwynhidwy]

When I suspect external flukes, which could be what you are seeing, I treat with two rounds of Prazipro with a 50% water change in between. I wouldn't treat with copper, I prefer Cupramine to CopperSafe, unless other symptoms appear.