I work in plastics recycling and we get sheets of acrylic in from time to time. I've often done weighted experiments on the sheets to see how far they'll bend before breaking. When you reach a certain critical point you can see crazing appear in the bend area, very much like in your pictures. And because acrylic is so stiff it dosen't need to be bent that much for stress marks (crazing) to appear.
It could well be, as someone's already said, that it is a diy tank which has been made using material with the incorrect thickness. If they were doing regular fin level water changes for example then the ebbing and flowing of the acrylic as the tank emptied and then filled again could have led to those stress marks over time.
It's a real shame you didn't post pictures before you bought it. I dread to think how much you paid for an acrylic tank that big, even a used one. Ouch!
