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Captive born yellow tangs can develop ich as easily as any yellow tang, it's just that if you get them directly from Biota, they are less likely to be harboring this parasite. If the fish first goes through a dealer's tanks, it is exposed just like any other fish.
Are you sure it is ich? This post explains how to post a fish disease issue if you want us to help diagnose and give treatment advice:
Aquarium Fish Disease Problem Solving Determining the best course of action for a problem with your aquarium can be a difficult task. Even advanced aquarists will often need to seek the help of others for complicated or unique issues. Our intent here on Reef 2 Reef is to offer expert advice...
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Jay