Hey Brian,
My tusk is over 9 years old and doing well. They are not very fast growers, so if this is for your 55 and it isn't overcroded, I'd suggest a nice medium 4" tusk. Very small tusks do appear in the hobby, they are tempermental feeders. Be aware that ornamental shrimp will be readily consumed. Mine has also developed a taste for turbo grazers, but I attribute his color to an occaisional escargot!
-Rob