A long time ago, I bought a really big tank at a yard sale. it had a metal frame that went all the way around and up the sides, and used a hard black material (think it might have been slate) rather than silicone to seal it. the rock also lined the inside of the metal frame on the bottom. Anyone know how old these tanks are? any more information in general? I'm ticked because my dad gave it away, but i'm still curious about it. I can post pictures of it later.