I used a cheap disposable box cutter. I know it sounds crazy but I found a box of 12 for about 3 bucks. I mean super flimsy blade and very small design. but the blade was small enough to fit between the edges of every tank I've torn down. And long enough to be able to rock between the edges as needed. I broke several of the blades, but they were so cheap that the blade broke away before too much pressure was applied to the glass. Kind of like a fail safe to keep the glass in tact.