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A bit more info from you will help in figuring this out.
How long was the damsel quarantined and was there any treatment used?
When you placed the fish into the main display, was it acclimated properly and did you notice any signs of distress or harassment from other fish?
Could you give some additional info on the main display tank? Age of set up, livestock, NH4
as well as other relavent info.
Do you still have the tang, damsel and clowns? In a 30 gal
tank this could be a major part of the problem.