You'll want to create a deep enough environment that plantlife can thrive and algae is discouraged from growing within your pond. You'll also want enough room to be able to enjoy the swimming area and completely submerge yourself. The regeneration zone should be kept shallow so that plants can access sunlight and oxygen easily. However, your swimming area can reach up to 6 feet or more.
Make sure to keep in mind that deeper water will be colder, and harder to clean. Consider investing in a vacuum in order to help remove any slime or mud that may make its way to the pool floor.
