I'm looking at you stock list and IMO
, there are a few things going on that are probably causing stress, which is what frequently leads to Ich, in Tangs, especially. It says you've got a 75g with four clowns and two different types of Tangs...
More than a pair if Clowns is generally not a good idea. It will very likely lead to territory problems and/or aggression. That can equal stress to the clowns, but potentially other fish as well.
Two Tangs is another concern. Powder Blues get big and most Tangs, in general, should be in a six foot tank. Powder Blues also become very aggressive as they mature, so even in something like a 180, they often do not play well with other tank mates. I'm not sure what your other Rang us, but I'd guess it could come down with Ich soon too. If its anything other than a Tomini, Kole or other "smaller tangs", then it shouldn't be in a 75g either. Tangs need lots of space to swim and graze. Small tanks won't kill them, but its nowhere near ideal, which usually leads to stress which usually leads to Ich.
I've never had to treat for Ich, so ill let others help with that. Copper or hypo are the only cures, so go that route and pair it with a fallow tank for about six weeks. You end up Ich-free and if future fish are QT'd prior to adding, this will not happen again.