For reference, I expanded roughly half a brick of EcoEarth and it was able to create a layer of about 2 inch deep substrate that totally covered the floor of my 10 gallon aquarium. For a 12x12 inch area I would think half a brick would be plenty for 2-3 inches of substrate. You can expand half of the brick by filling a good sized bowl (think salad bowl) with dechlorinated water and then standing the brick up vertically and leaving it for about 10 minutes. Then the brick should be soft enough for you to break off about half of it and let that half sit and soak until it totally crumbles apart.