The salt dip creates a pressure differential on the cellular level, and external parasites can't handle it, so they die. Cecil uses the salt dip a lot when he's transferring fish from one BOW to another on his place. It's O.K. for smaller batches, but it'd be a PITA to do it on larger batches. You need to have 3 tanks set up. One that the fish are in, another with the salt dip and a 3rd with clean water. Here's what Cecil posted a while back:
Dip the fish up to 30 seconds or a minute or until it turns on it's side. Weigh out 1.25 lbs. of salt per 5 gallons * of water for a 3 percent solution, which is about the same as saltwater give or take. If the fish goes on it's side, immediately put it back into the pond. Use water from the pond.
*A cooler would be better than a five gallon bucket as the fish won't have problems keeping horizontal if it's a larger fish. Don't guess on the gallons or salt level. Measure as precisely as possible.
I would use non-iodized salt from the grocery story, and stay away from water softener salt. Many times water softener salt has an anti-caking agent know as yellow prussiate of soda (sodium ferrocyanide)that can produce hydrogen cyanide in the presence of sunlight, which is toxic to fish.