For you to truly get iCk out of you tank you'd have to run it a very low salinity for 6-8weeks, remove ALL live stock, remember this is a parasite and the name and most of the common cures don't work because the name and cures came from the fresh water hobby not from marine. The fish(s) that have it it's important that they are eating if they stop its not good. The feeding of foods soak in garlic, Vit C, Amino can support the fish immune system but will not kill the ick in the tank just makes the fish (maybe) a little less stressed and not so prone to the ick.
Since ick is a parasite and has layed eggs its in you sand, rock, EVERYWHERE. I've saved my Tangs by food supplement and a fresh water dip but I did the dip when they first showed signs not when they were as spotted as yours is. Hope it helps.