Guys I know a thing or two about these things. Too much here on this thread has been over simplified. I'm not going to get into a debate over it but, ill give you my experience/my opinion on this.
Looks like the tank was built on a home made stand. The owner if you noticed declared how strong it was but, did not say it was professionally built. Odds are that it wasn't. Also the potential mfgs were mentioned with some of them being more likely to break. I can tell you that any tank on a stand that is not built level can crack like that. Now it sounds great that it could be as simple as replacing the panel of glass, doesn't it? Well it's not that simple. If that tank was sitting on a cockeyed stand for years until the weakest point gave way to a cracked panel.......how well can you be confident that the other panels weren't stressed? It's likely the seams/joints were weakened while sitting on this stand and perhaps you won't get a cracked panel next time....it will be a split seam.. The other thing to think about is that if you build a stand or even have one made its not going to be the same contoured shape as the original. The tank then has to resettle to a different stand. I have seen subsequent problem with this in the past.
It's a big risk regardless and really shows the reason why mfgs warranty often only apply if purchaser uses their stands. There is a lot of weight/force involved here. How bad would it be in your house with 300 gallons on the floor?