You need 624 cu. inches of dirt for each inch of dirt you want to add to your tank. According to Home Depot, a 32 quart bag of MGOCPM contains 1.25 cu. ft of dirt, which comes out to 2,160 cu. inches, so the large bag will give you more than you need. As for your 19 qt. bag, it should give you enough dirt to give 2 inches of dirt in your tank. Of course, this is assuming you don't sift the dirt. However, even if you sift it, I think you'll still have more than enough to get you one inch of dirt in your tank. Try it out, and see where it gets you. If you need to add a little more, you can always just buy an 8 qt. bag for $5.