are you sure this is a crack? or just scratches, from cleaning or rocks etc?
if this really is a crack where is it? is it at the bottom or top. if it is at the top, lower the water level to relieve the pressure.
what you can do to hold it is get a sheet of glass approx the size of the crack, I'd recommend 6"x6" x the thickness of your aquarium glass and silicon it over the crack. you dont have to use a lot of silicon, just enough to make a good bond.