quarantine the gt right now, drain and sanitize everything, house the gt in a bucket and try to treat him that way. at least your other fish wont die too.
Posts #12 and 13 from the OP would suggest that things are under control now, and Aquanero probably zeroed in on the issue. I don't think draining and sanitizing the tank would do anything other than stress out all the fish, the GT included. Overreactions can be as problematic as inaction.