Petsmart has the tanks as low as $10.99 with your Petperks card, so that's $14 you can apply to other items in your cart such as decorations or food.
Petco has some Marineland tanks as part of their $1 a gallon promo. So a variable Mm with overage to apply to your cart as well.
Meijer has them starting at $19.99, so free with this manufacturer coupon. Meijer doesn't pay overage, it autodeducts down to the item price. pookiedeal!
Aquarium Adventure had them as low as $15.99, so a $9 mm with overage to apply to other items ymmv not all cashiers there allow overage.