imo no fish is really immune to ich just a healthier fish can fight it off easier and some fish just get ich if you look at it the wrong way (powder blue tang)
but if you are going to remove all the fish and treat and put them back if you stress them while putting back you might end up with it all over again it will always be around its just less dramatic if you Qt fish first make sure its doing good before introducing to the display will lessen the amount of ich parasite or chances of other parasites entering display .....
Myself i think every tank has ich..some more then others some to the point they dont see it for years until a major temp swing or a bully comes along and stresses out a fish to where its slime coat weakens and ich can act on...
this is just my opinion here
Wow sorry for the novel of a post
and inverts are not effected by ich
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150 mixed reef 14 fish 4 shrimp 50+ snails
55 sump long spine urchin 20 snails
Shawn
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