if you manage to catch it,
dip in myazin 5 times the normal strength for an hour or 2. observe him, if he didn't cope well. return it to tank.
if it doesnt help, dip 10 times the strength. i treated some of my fish via that method. however, theres some risk involves.
or like what terry suggested, improves the water. do a water change, feed lesser and see how from there.
if things get worst, that fella might go blind. so act fast.