It was a Glasscages 180g 48X24X25. Many reasons why it could have broke:
1. Oh, probably the most important factor was that one of the top glass braces was chipped pretty severely by my father being a dumb a$$ and dropping something on it. The main part of the brace was intact but the top layer had to be siliconed. This was on the same side and same area where the crack started.
2. Mishandling when moving, not supporting it correctly, even with the suction cups
3. I built my own stand, which was dam level, but could have been off slightly
4. The tile floor underneath could have been not level
5. Possibly the changes in temp from 93F or higher under 3 MH's to air conditioned 68F at night repeatedly could have expanded and contracted the glass, especially if already chipped. It also could have expanded and contracted the wood in the stand
6. There may have been micro cracks, or I didn't repair the cracked brace sufficiently which eventually fully cracked the whole 1/2'' through
7. The construction of the Glasscages tank was not superb, the black trim was coming undone and was flared out on the bottom right
8. Glasscages top bracing wasn't completely surrounding the top, it had gaps around the overflows
9. I didn't mishandle it except when moving it and never smashed anything i.e. LR against it
10. I used the purple foam from HD to add cushioning under the tank. It was stiffer than most foam padding
So I would say the cracked brace started the crack and the expanding/contracting finally gave way.