remove from tank asap, tub in fresh cold dechlorinated water which contains 2g/l salt, test tank for high ammonia and/or nitrites check ph, if ammonia high continue tubbing in cold water (dechlorinated first) changed daily with 2g/l salt, add almond leaves if possible, until tank under control make sure filtration adequate, if nitrites high continue tubbing same as with ammonia but also do daily baths with methyline blue (half fish dose, directions on bottle) if lethargic tea baths can also be done as well.
do not add back into tank until issue has been dealt with, this includes the nitrogen cycle working correctly.