Possible Gill Flukes

I have had two GSPs for about a month. One of them started behaving lethargically and turning dark grey on the sides. This has been going on for the past two or three days. I added marine salt to the tank, since they are brackish fish, and he seemed to perk up briefly. However, he is much worse today. His coloring is greyish over his back and sides, his face is black, and he is breathing heavily. I saw a red strand about 1/2 cm long coming out from each of his gills.

I called my pet store and they said it was gill flukes and I used Seachem's Paraguard to treat it. So far he's still the same (only been a few hours). Does this sound like gill flukes or something else? What else can I do?

The other puffer still has great coloring and swims around actively.

I keep the tank at 76 degrees with a ph of 8 and have about 1 cup per 10 gallons of marine salt. I also treat the water with Seachem's Prime.

Any help would be greatly appreciated. Thanks!