How are you treating for ich in quarantine?
If you rid it of ich in quarantine, then there is still ich in your display... so you're just adding it back into an infected tank and therefore it'll develop ich again.
You have to remove all fish from display, leave it fishless for 8 weeks, and treat all fish for ich in quarantine to be sure that it won't get ich again... even then it's not a guaranteed solution since there's a large margin for error.
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