The best way to make a straight break in glass is to keep the pressure over as wide of an area as possible. With smaller pieces of glass, there are wide-jawed pliers that do the job very nicely. With larger pieces (and 2' counts as "larger"

), hold the piece on the straight edge (usually a table), or, better, have someone else hold it, and then use a straight piece of wood or metal, placed with the edge at the score, to press down on the glass.
I've never cut aquarium glass, but I've done stained glass, and this has always worked.