Need ich advice

[S0K]

Hello all this is my first post lol

So about a week ago I bought two tangs: a yellow and a kole. They seemed so happy and healthy at the lfs so I didn't QT them (I know, shame on me) and just put them straight in my 125gal DT. They both seemed happy and problem-free and since day 1 eat everything I put in the tank (frozen Mysis, pellets, SeaVeggie flakes). They get along fairly well and often swim together although the yellow seems to have crowned himself "king of the tank". The only other fish in the tank are 1 ocellaris clown and 2 green chromis.
Anyways here is the problem: yesterday the kole had 2 spots on one of his pectoral fins. Today the spots have spread onto his body. None of the other fish have spots but I know "if one fish has ich they all have ich". My question is do I need to do anything about it? Thing is I don't think I will be able to catch the kole to treat it in a separate tank due to the rockwork and can't medicate my DT because I have inverts.
Here is my "plan" please tell me what you think of it (don't hold back)... I'm thinking so long as he is still eating and behaving normally I leave him in the DT and continue feeding him pellets/Mysis/SeaVeggie flakes soaked in Garlic Xtreme. My hope is that as he gets settled into the tank his immune system will grow more resilient so next time the ich rises from the substrate he won't be as prone to become a host. Does that make sense? I also thought after these first spots fall off I could vacuum the substrate as a means of reduction of the parasite... HOWEVER if the spots do not reduce and/or start to affect his breathing will that kill him?

Sorry for the long post just wanted to include all the details.