Your weekly water changes are going to help tremendously in hopefully keeping parasites at bay. As dirty water and poor housekeeping with your pond will certainly invite them in. Ponds are naturally filled with bacterias and such, and when your fish are healthy, they are not affected by them. It's when your fish are ill that these critters take advangtage of a weak fish and cause havoc. Parasites, of course, will target a healthy or sick fish. They don't care.
That being said, you can have a perfectly clean pond and can have a parasite hitch a ride on a new fish you may have introduced or even a plant. You'd be surprised the way in which parasites can come into a pond. Heck, even a bird pooping as it flies by and it happens to land in your pond can be a problem if the thing is carrying a parasite or bacteria.
Using salt is a catch-22. There are folks that continually have a certain level of salinity in their ponds. I prefer to use it only when I have a need, because bacterias and parasites are said to become immune to salt if it is used continuously. They essentially develop a resistant to it. Kinda like using overusing antibiotics in a human, so to speak.
There are parasitic and bacteria foods that are made by several companies, Jungle is one of them. I feed them to my fish in the early spring as they come out of hibernation and then again in the thick of summer as a preventative. They are definitely not cheap foods, but I don't consider a dead fish cheap either.
Don't go crazy worrying about parasites, rdk. Keep a solid cleaning regimen and you will be doing a lot of good.