More experienced aquarists will probably answer after this, but here is my two cents...
Your tank seems stocked pretty heavily, and the rest of the fish almost certainly have ich to a certain degree. The ich goes through two phases in its lifecycle, a dormant phase and a parasitic phase where it attaches to a host fish. They also reproduce like rabbits.
If you slowly increase the temperature a bit, you can speed up the lifecycle, and if you slowly lower the salinity a bit you will make conditions more favorable to the fish and less favorable to the ich.
Personally, I wouldn't add another fish for a long time, if ever. Your whole tank is a delicate balance bwtween the ich and the fish, and once the ich population gets beyond a certain point, the situation gets exponentially worse. The ich cannot live through the parasitic phase without a host, so the fewer fish the better.
My advice: Don't add any more fish, slowly increase temp & lower salinity, and try to maintain excellent water quality.
Good luck!