When a fish moves only one gill plate it could be a sign of Gill Flukes or a possibile bacterial infection. Try adding aquarium at 1 tblsp per five gallons of water and see how the fish is doing in a day. This could be a water issue. How often and at what rate do you do water changes? As anything new been added to the tank with out being QT'ed in last month or so? If flukes are the cause Tetra Parasite gard is a good choice. If it's bacterial you will have to go another route let's see what happens after you do a large water change and add the salt.
