Serious Crazing!

I would be concerned honestly. While it isnt all that common, acrylic tanks can burst. Signs of impending breaking is bowing and crazing. It may never happen, however it might. Also as mentioned above the air bubbles in the one pic is concerning

 

personally never owned acrylic, but had a friend who almost bought a used tank, guy said it was fine i looked and pointed out the crazing and asked for it be filled on the spot, i kid you not he the filled the tank and he was sweating bullets as it filled i saw the bowing and then in a instant burst. but i mean the crazing was pretty bad and there were air bubbles in seam, but as mentioned it could hold together for years to come or burst the next day this is from years of fresh water exp. i saved my friends arse by doing so but i propally ticked the other guy off

 

imho it's what will hold a huge investment so why worry about it forever ....which you will
same old saying about stuff in the fridge comes to mind
"When in doubt ...throw it out"
personally i wouldn't hesitate to use it for freshwater but i don't think i'd use it as a beginning to a very expensive investment