You are confusing the efficiency of aeration with need to have depth in a pond. Aerating from the bottom up is more efficient with depth but you don't build a pond 20-30 feet deep to aerate, you aerate because the 20-30 foot depth has a problem holding oxygen. You want to have reasonable depth to have volume to create stability in a pond and also prevent aquatic weed invasions, but you also want the depth to be optimal for a healthy pond. In the SE you don't need to go any deeper than 10-12 feet and you can get by with as little as 7-8 depending on the size. Anything greater than 15 feet is not necessary in any size pond. It would help to know what size area you are talking about.