I had a koi pond for 35 years. And it is all about filtration, filtration, filtration. If you use quality filtering, including U.V., parasites should never have been much of a problem.
In 35 years, I lost 2 koi. Only two. Plus there is a tonic like chemical that you can use which should help. If you keep their immune system up, they should fend of any parasite.
Does you pond have a plastic liner?
How big is it but I would let it sit for several months depending upon size and type.
