Ask if it is tempered, and also if it was baught as a "reptile tank" or an aquarium used for reptiles. Also, ask how long there were reptile in it. Over time, the silicone will dry and become seperated from the glass,thus making it not water tight. I have a bunch of reptiles and know exactly what you are talking about. Oh, and if there was sand in it for a while, it can leave a perminate hase on the glass. And some lizards will leave micro scratches in the glass from their claws.