Ich is a parasite,it gets into the fish and irritates him. He is trying to knock off the parasites.
I recommend that you start reading and researching this hobby. You are keeping alot of fish for such a young tank. Please slow down and don't add anything else. There is a possibility that the other fish have or will have it as well. What happens is the fish gets stressed and it loses it's slime coat which makes him susceptible to the parasite's attack.Saltwater fish,especially regal tangs,are prone to diseases when kept in cramped quarters. This tank should have matured before adding tangs and an angel.I would keep a close eye on the angel..he would be my next suspect for disease. There is a disease and treatment forum here...I recommend that you start reading up on ich,hypo-salinity,and quarantine. The fish needs to be treated or it may get worse and kill him. If it is a mild case,and you can reduce his stress, you might try adding a little garlic or garlic extreme,and Selcon to his food. This boosts the immune system and gives him a chance to ward off the infection. There will be a period where the ich drops off and the fish looks better. The ich parasite is just regrouping and multiplying,and can come back worse than before. It is best treated with hypo.I would also suggest that you rethink your tank...remove at least one of the tangs.
