IF you must keep the plants
(I know, they're pretty and you spent more on them than you did on fish...) here's my suggestion.
Pull the plants and place them in a separate container. PP Bomb the hell out of their water to kill off any of the nasties they probably infected your pond with in the first place.
While the plants are being nuked by the PP (it won't harm them) treat the pond water with Dimlin until the problems with parasites have been resolved. After the PP goes off (turns the water brown) you could go ahead and hit them with a dose of dimlin too just in case. Then you can place the plants back in your pond safely so they can provide a nice host for the next parasitic infestation brought in by a bird or a frog that may happen by...