The guy assured me that it is a fish tank, that it was used for fish for a long time, but he did not seem that confident and neither am I. It's about 40-45 gallons (long). The thickness of the glass measures about 1/4 an inch. (seems thin but I'm hoping to hear otherwise!) I currently have it filled to the top sitting outside, so far so good. And if this is unfortunately a terrarium is there anything I can do to make it a safe aquarium? Here are some pictures.