Depends on the leak. If the glass is cracked in any way your rolling the dice if you decide to fill it up. It might hold just fine, it might leak, it might outright shatter. If a seal is leaking you can go through the fun job of resealing the seam. Buy some aquarium silicone for this. Strip off the old silicone and redo the entire seam with the new aquarium silicone. It's a pain and it takes time but a tube of aquarium sealant costs like 3 bucks so its a lot cheaper than buying a new tank.