Hello; I have repaired tanks with one pannel of glass severly cracked (no pieces missing) by cutting a piece of glass that will fit over the entire broken panel. I secured it in place with large amounts of silicone. Running a bead around the edges of all the cracks and a bead of silicone around the peremiter of the entire replacement section (silicone on both pieces of glass). Carefully press the new glass onto the broken side untill the silicone flattens and spreads. On the inside where the broken edges are exposed you may need to coat the edges with silicone or another layer of new glass on the inside to prevent leaks and future cutting of yourself or the fish. (note- If you can find a safe way to do it, you may want to try to smooth the sharp edges of the broken pieces before starting a repair.)(perhaps with a dremel tool and a grinding stone)(Bits of glass will fly around, so wear eye protection and heavy gloves.)I have a 20 long that is over 30 years old that I repaired this way. It is an ugly repair, but still holds water. I sometimes use it as a breeding tank or a holding tank.