Sorry to say but I dont think this is a good Idea in the long run for poly's. 8inch is just to little in my opinion for a grown poly even with the larger footprint. If u get alot of polys in a pile someone will be out of the water. A full grown poly would be strong enough to jump out of an 8 inch tank even if u have something ontop to hold it down u dont want anything to heavy that might fall in if the poly jumps and hits it. Just my thought u do what u want tho its your tank