Really, no one has replied? Result will depend on cause. If from an accident, then it's a matter of how much glass thickness is remaining. If it is an intrinsic defect just appearing...then their may be a catastrophe in the making. In the sense of an aquarium, I know not what to do. In a glass window, you would drill a small hole at the tip of the crack to relieve the stress and apply a patck. In an aqurium, I don't know. If it is tempered glass, the above will destroy it and I am clueless.