the salt dip will kill most bacteria and external parasites but wont necessarily kill internal parasites. I like the freeze method. If it can survive being frozen solid, good luck killing it.
Also, try to find a better feeder. Goldfish have a host of problems associated with them that goes far beyond the parasite risk. They are not very nutritious, they are high in fat, and high in thiaminase that will cause other health problems down the road. Just saying, there are plenty of easy to raise better alternatives