did they net him or dip him out with a bucket when they caught him???
look at him real REAL, close if you see anything on his anal or dorsal fins like white spots ,speckles, or anything id qt him in a quick hurry fast fashion!!!
if your fish is nettes its kinda stressful to the fish, cause it messes with there slime coating alot, and thats what protects them from parasistes(helps any way) and if thats interupted than thats just kinda down hill from there
sounds like you may pull it off though.
how long have you had him how big is his permanent tank, and does he have anytank mates?>??