Good luck with everything, I can not imagine what it's like to have a tank crack like that.. I'll bet it's the tank and that all the glass pieces are not square on all sides. I would take it apart and check every part just out of curiosity and if it is the bottom pane of glass have it cut narrower or get a new piece that is narrower and get a new front pane. some times when they cut thick panes of glass the end will run in ward and that is what I suspect happened to that tank the last little piece ran toward the center and is not that noticeable and was missed at the manufacturers. If I were there I would get it off you as I would like a 6 foot tank that is what my stand is built for anyways. I over build things my self..

Anyways good luck.
Bill