SurferGirl,
I admit I'm conservative when it comes to fish-keeping but that's because I learned the hard way that too many fish in a tank is bad! I had 2 tangs in my 45 a few years ago.....they both died from ich caused by stress caused by being in a tank that was way too small for them. I believe you have the potential for the same thing to happen. Most experienced hobbyists on this board will suggest the minimum tank size for a tang is a 75. Keeping two mature tangs would require, at least, 125 gallons. You have two tangs, two clowns, and now, two coral catfish in your 55. I think you're overstocked......and no matter what else you do with your tank, I wouldn't add another catfish, or any other fish, to it.
Trying to return one of the tangs to the store where you bought them, would be a good thing to consider, unless you have near-term plans to get a larger tank.
Jim