I have lost 4 out of my 5 goldfish. The last one I noticed was very lethargic towards the end and hanging at the surface of the pond barely moving. I put him in a separate tub and noticed he has what looked like little white strings hanging around his mouth and some on his body. From my google searches I think it may be anchor worm. Has anybody had run ins with this or other parasites in their pond? I have one fish left in the pond (8ft diameter stock tank) and am considering taking him out and treating him in a separate container if needed. I read that Potassium Permanganate can be used to treat them. However I am open to whatever will work. I assume the whole pond should be treated.

Parasites are ubiquitous to ANY aquatic environment. Since they are opportunistic organisms, they will only affect those fish that are already weakened usually because of poor water quality. You can certainly treat the entire pond but be aware that the parasites will return as their presence is normal and natural.

In which case I will, have to monitor how the fish are doing and treat the whole pond accordingly?

See this from SARC

https://srac.tamu.edu/serveFactSheet/171

Sounds like the pp listed in the article treats a bunch of parastie problems.