That's not a manufacturing defect. That's from an impact. You might have trouble returning it. Might be hard to prove that it was there when you purchased it. I have a 28 gallon that I ran for years with one just like that, with no issues. You'll probably have several people chime in here and say it's unsafe, don't fill it. (this isn't the first time this has happened) Ultimately, the only way to be sure if it's a problem or not is fill it and use it for a few years....but that's a lot of water!