I saw on youtube a guy put an old plastic (clear) cd case over a large crack in his tank. It worked. Heres what he did, although I would use something a bit classier for your tank. 1. clean the area, and make sure it is dry. 2. clean your patch, dry also. 3. take clear, silicone epoxy or caulk that is water proof (make sure it states on the tube it is safe for aquarium/fish) cover the entire piece of patch and I mean the whole thing. 4. quickly stick it to the outside of your cracked tank and hold till it is secure. 5. let dry for the suggested amount of time according to your type of adhesive. 6. after letting it dry for a couple days, fill with water and set aside to monitor for any leaks and to make sure your patch has worked. 6. if all is well, acclimate your fish again since you wil be adding new water. hope it works, it may not be the most prettiest way to patch a crack but it will work.