I hate it when stuff like that happens. Why do you think you have parasites? Beyond water quality, there are other things that cause fish to die. Where are you located? Have you added anything to your pond recently? Are your friends using it as a wishing well?
Assuming parasites, before you toss all your plants, figure out what parasites, vectors and where they came from. Are they external parasites like dragonfly larvae? Or internal? A lot of internal parasites have one part of their life cycle in snails. Did you inherit snails from someplace?
