I would not try to draw them back together. I have several older tanks with cracked frames. It seems as though the plastic shrinks with time, gets brittle, and cracks. I don't think the water pressure pushing out on the glass is what causes them to snap. In other words, I don't think that your tank is now wider than it was. I think the frame is just smaller. I would just remove some of the water to relieve some pressure, and repair it where it is. If there's a small gap, I'd leave it. That's just me though.