Without a scrape and scope of one of the living fish, not much to go on, but PP will take care of most parasites and you have it available. I would suspect the incoming fish brought in something like costia, which will kill quickly, so go for the PP for starters. Follow with Prazi as soon as you can get it in case the problem is flukes, which PP is not as good at taking care of.
For the PP, for most established ponds, there is enough organics in the pond to consume a large amount of PP, so start with 4ppm, adding slowly with lots of aeration. If it doesn't keep a purple color on the first try, then wait a day and do it again. The dose for the second day needs to be determined by the amount of time the color stayed purple. If only 1/2 hour or less, do the 4ppm again, if 1/2 hour to 1 hour, go with 3ppm, and if over 1 hour, then just 2ppm. If the color stays for at least 4 hours, but better 6 or 8 hours, then do not redose.
Zone 7 A/B
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Richard