Ok clarafication on the tangs...
I have a shoal of red sea purple tangs. They grow very large.. far too large for a 55 gallon. I kept a single one in 100 gallons and its behaviour when released to the 220 system was remarkably different. Less nervous, not as aggressive, and constantly drifting around the tank grazing rather than darting around looking for hidy holes to get into.
Regal tangs get even larger. I have considered a regal for my system lately as it needs a good blue colour to it. HOwever i am not comfortable with adding another tang with so many in my system already (and this will be the 2nd largest tang in the tank once its grown).
Yellow tangs dont get any smaller thanthe purples. (They are basically the same fish with a different paintjob

).
Lok for a bristletooth/yelloweye tang if you really wantone but even this size of tank is pushing the limits of comfortably housig one.
Large angels are completely unsuitable for a 55 gallon, you are better off with dwarfs
Maroon clowns are fantastic clowns, probably my favorite but.... and there is a but....
They grow very large.. much larger than even i took them credit for. I saw a fully grown pair in a shop recently and was astounded at their size. They are also quite pugnascious when they want to be too. Considering the size they can reach and the power they can posess i would urge caution with these littel fish you see in the shops as they can grow 4x larger than this and most occupants will not be able to defend themselves should the clowns want the tank for themselves.