Define "crack". I know what I think of, but I've seen cracks that I would call chips, so a photo will help. Assuming it is a small crack you may have the option of drilling a larger bulkhead hole that will remove it. If not you have a couple good options, and a couple bad ones:1) Replace the panel.2) Turn it into a reptile tank.3) Drill enough holes to remove the cracked portion and patch over it. This won't be pretty, but may be functional depending on the circumstances. This depends on among other things the thickness of the patch, its proximity to an edge, and your personal acceptance of risk.4) Replace the tank.I would not personally use the tank with the crack still there, patched or otherwise. If on the other hand it is more of a chip there are easier solutions.