Why my tank cracked?

[taybelZ]

Just wondering does anyone know how many KG a 48 x 18 x 18 tank should be able to carry in terms of mass? I recently setup a tank and it cracked on the bottom in the middle @ going the whole 18inch. i haven't measured the glass thickness but i bought it from an aquarium shop. Also i weighted my gravel and it weights 50 KB, the tank stand structure is stable as well. Just thinking is this just a freak thing to happen or my gravel too heavy? The gravel is some iron based gravel and 1kg of it compared to standard gravel looks like a 1/2 a cup.

[Francisco_Borrero]

Well, of course I don't know why it cracked, but these are my thoughts.
Your tank sounds like a 75gal or close to it. If it was a fish tank (as oppossed to what is sold as a critter cage), it shouldn't have cracked because of the gravel itself.
Rarely, if a large rock is placed in the center, and something sharp (even as small as a hard pebble or grain) is just exactly under the pressure poinjt created by the large rock, the glass can break. This is unusual but can happen. This is why it is recommended that large rocks be placed directly on the glass, not with sand in between.
In your case, with gravel distributed over the bottom, it is extremely unlikely that the gravel itself did it, no matter if heavier than other gravels.
It sounds as if : 1- the glass was already weakened somehow, or 2- The surface where the tank sits is not level/even, even if the actual legs of the stand are.

Sorry for the loss. Cheers, Francisco.

P.S. Occassionally, the glass of old tanks can get weakened if: 1- left in full sun for long time (as in, all summer or more in backyard/porch), 2- left at freezing temps and some water outside (as in, even a small amount of water that freezes and then thaws).