Here is a chart letting you know which tanks are tempered on the bottom pane.
http://www.aqueonproducts.com/assets/011/19107.pdf">http://www.aqueonproducts.com/assets/011/19107.pdf</a>
Also a lot of the those $1 Petco tanks are now Tetra brand not Aqueon. The Tetra tanks are tempered all around.