I would try paraguard by seachem. But you would need to treat in a separate tank. Is he still eatting? Do you have ich in your tank? Have you seen him scratching on rocks or sand? Good luck keep us posted.
Started to bleach due to the fluctuations of your tank. Once a nem bleach its almost impossible to get the color back. Keep a stable tank and feed it more if it will take. Hopefully the color will come back. Good luck.
I say your tank size is fine. If you really want to get another clown I would catch the solo one. Send him to the sump for a week or two. Let the new one explore and eat and get happy. The reintroduce and cross your fingers or fins. Good luck.
Wow advice giving is not good. Many things could be going on. But first start out with how long your tank been setup. What is you water test reading. Your lights? And most important what kind of anemone?
Anemone do very bad in shipping. I say 50% survivor rate. Hardy ones will do...