The best rule to remember when buying tanks from Craigslist or mfk (used or new) is to ask the see the tank completely full of water. Then carefully check along the bottom, sides and top for any signs of leaks or damage. Check under the stand too for signs of water damage.
I've avoided buying a couple bad tanks because of this. One guy tried to sell me a tank that was half full. It was a 240g for only $400. When I asked him to fill it up completely he said he was too lazy to fill it. I later learned that it leaked pretty badly and he left town right after selling it. Another guy tried to sell me a 300g acrylic tank that had a bad chip in the corner. He said it was fine, but there was a lot of water damage under the support of the stand. So i avoided two bad deals bc of careful inspection.
Chips can be an easy fix if you know how my 75 has a chip and it holds just fine
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