They way i was explained on how to do a freshwater dip for zoos is as follows. I had really good results.
Fill up a 5 gal bucket with 3 gals of fresh ro water. Add 5-7 drops of iodine per gal of water in the bucket. Remove your zoos from the tank and place up side down in the fresh water. Let sit there for 10-15 mins. After you wait the time, shake them vigourlsy to remove the dead organisms. Then place them back into the tank. They will act better under actincs at first, then after 24 hours place your lighting back to normal.
All my zoos had closed up on me and I did this procedure and they seem to be comming back out and showing more. Zoos depend on iodine supplements in the tank.