I bought my 55 gal from petco's 1$/gal deal last month. I've been using this fixture: http://www.drsfostersmith.com/produc...03&pcatid=3803
The fixture works great, but it does not fit exactly well on my tank. The fixture just slides around and is a hassle when I'm doing maintenance. I am also using a glass canopy.
I think the dimensions are 48x13x18.