The T-12's may not physically fit in the fixture (it's wider tube than the T-8, though the pins are the same). I also learned that a T-12 fixture may not like T-8's.... it's all about the ballast: Some will run both T-8's and T-12's, some will just do one or the other.
I made a dual light "fixture" with a Lowe's 2xT-12 shop light, three 48" lengths of wood (pine, I think), and a tube of liquid nails. I disassembled the fixture, screwed it to one wood panel (to be the top), reassembled the fixture, then "liquid-nailed" the other wood boards sideways to the above board to make a "front" and a "back" to hold the light up over the tank. Pretty "ghetto", but works fine. A little wood stain, and a little more effort, and I bet it could look OK.
-Ryan