and to answer your original post, it's very uncommon if your mandarin does have it. If it does, only known treatment is copper. Quarantine and copper treat. I go more holistic myself. If any fish of mine has ever contracted ich, I increase feedings, use garlic and vitamins, and hope the fish fights it off. I firmly believe that catching and quarantining can do more harm and stress to a fish than just leaving well enough alone. You'll find various arguments and opinions on the matter for sure.
Good luck, hope your mandarin doesn't have it.