are you sure its a crack? and not a deep scratch??? it wont look very nice, BUT if it is a crack and it is glass, i think you can get apane of glass silicone it on to the crack, then silicone round the edges making a air tight seal, and then it should be ok
I've seen a couple tanks at a petstore once that have silicone on the crack, it doesn't look that great but I would do it so you don't come home one day with water everywhere. 125 gallons is a lot of water especially when its all over the floor.