I purchased a 20g tank on OfferUp and although the guy told me that he cleaned it, there were a few pebbles (they used it as a freshwater tank) at the bottom but barely any. This might've hidden the about 4 inch crack that I just found while cleaning the pebbles out before starting to set it up. The crack doesn't go to the outside, it's just on the inside. I'm nervous about putting sand, water etc in it. What should I do?