I would not use any other treatments in your display tank. You can set up your 55 and put the fish in there with out cycling. Here is an article
Reefreaders - Quarantine Now! that should help you with setting up your hospital tank. You will have to test the water and use water changes to manage water quality. With one fish in a 55 this should not be a major chore. Get a sponge type filter and stick it in your
sump overnight to get some bacteria growing on/in it, then put in in your hospital tank.
With such a non-specific problem I don't think that I would use any treatment. Instead I would feed a variety of foods soaked in vitamins and garlic. This will help strengthen the immune system and hopefully allow the clown to recover. A hospital tank also would allow your fish a place to relax and not worry about predators.
