fish died

[tablesalt]

Did you notice any signs of ick on the other fish before you actually found the fish dead? How long did you have the mandarin for, and was it eating prepared foods?

You will have to set up a quarantine tank and leave your diplay tank fishless for awhile to make sure all of the ick is gone. Powder blue tangs are known to be very susseptable to ick, and it is very likely that it was the culprit of bringing the ick into your system. The best thing to do is to quarantine your fish before you put them into your tank.