So... this is not a fun post, but I think it will have a positive outcome.
I was sound asleep last Thursday when I heard my wife scream from the kitchen that there was water running in the basement. I obviously panicked and jumped out of bed, and ran downstairs to the sight of water gushing out of the right side of the 420.
The seam between the front and side glass split about 2/3 of the way down. I was able to open the emergency drain on the back of the tank below the break, saving around 100 gallons and keeping the fish alive.
We then began the tedious project of pushing as much of the water into the sump pump. There was standing water on most of the basement floor. My best guess is that the tank was not perfectly level, and the uneveness caused stress on the front glass. This is just a guess. The tank was used, and it could simply have been age. I took a few days to think about what I wanted to do next. I thought about just taking it down, but I didn't want to give up too soon.
I decided to try to fix it. I was able to get silicone into the seam as far as it split, and used ratchet straps to pull the glass back together. I also releveled the tank while it was mostly empty. I didn't trust the silicone to hold on its own, so I built a frame for the top of the tank (and am planning on putting one around the bottom of the tank as well.) I cut it to fit snugly, then used ratchet straps to pull it tight before I screwed it together.
At this point, the tank is about 3/4 of the way full, and no issues. Only time will tell. We are a long way away from finishing the basement, so I guess I will wait and see. This was not a fun experience. And if it was my fault, not a mistake I will make again.