Lugols is good, but also deadly. I'd only add a drop or two in a gallon of freshwater. I used to dip for about 3 minutes, and rinse/pull/cut off any zoo's that don't look healthy. It is better to give up a couple so so zoo's than loose a bunch. Brush the rock with a toothbrush if it looks slimey and rinse in the fresh water. When you place them back in your tank put them in a place with GOOD flow. You'd be surprised at how much flow zoos can take and will actually thrive.