Usually when this happens, the tangs or any fish are looking for nutrients they are not getting in their diet. Soak ALL their food EVERYTIME in a multivitamin, Garlic, and Amino acids and Omega 3's. Feed them multiple times a day, especially Nori. Some examples are Selcon, Garlic Extreme, and Brightwell makes a good multivitamin. With my 4 tangs I would go through at LEAST a Half a sheet of Nori a day, and after a while you will see all of your fish wanting the Nori as well. By spreading out their diet and after all these soaks, all your fish should be getting exactly what they need in their diets and will not be looking to supplement themselves by nipping corals. Although some would say eating Xenia is a good thing!!
Anyway, if any of this sounds like too much trouble, you can make enough to last for like 2 days in a tupperware and store in the fride. I buy all the variety of cubes of frozen, the best quality Flake food I can find, and Nori from the Oriental Section of the grocery store. I then throw in all the different cubes, cyclopeeze, phyto, zooplankton, Rod's food, tear up the nori into flakes, and just a tad of the flake food, and the soak it with all the above mentioned soaks. You can mix it all together or seperately. With all this mixture your fish SHOULD be getting every nutrient they need and you will have happy, colorful, active fish!
WIth this recipe and of course good Tank husbandry your fish will be bulletproof! I have not had a fish die, or get a disease in over 3 years. Since I started feeding like this. I DO NOT acclimate any new fish, as I am with the thought that it only stresses the fish out more causing disease or death and I have never had ICH or anything like that.
