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Use aggregate or gravel that does not contain any additives or large portions of organic matter that has not decomposed. Make sure the aggregate has not been in contact with animals, as you do not want germs or bacteria to get into the water. Use fresh water to fill the plant zone. Make sure the water runs into the pool area easily so the plants can help to filter the water. Check that the piece of black liner you are using as a barrier sits upward in the water, preventing the plants from drifting into the pool area. Waterweed and hornwort are both great options, as they release a lot of oxygen into the water. You can also put in aquatic plants like sedges and rushes on the perimeter of the plant zone to keep the area oxygenated and contained. Water lilies and other floating plants are great options for the plant zone, as they encourage the growth of microorganisms that will keep the water healthy and clean. If you are using plants that have roots, shovel gravel onto the bottom of the plants so they stay in place in the plant zone.

Summary:
Put 3 to 6 inches (7.6 to 15.2 cm) of aggregate or gravel in the plant zone. Fill the plant zone with water 1 foot (0.30 m) below the top edge. Place oxygenating plants in the plant zone to keep the water healthy. Add in floating plants to provide shade to organisms. Anchor the plants with gravel.