Plants can be great for assisting your aeration system, absorbing excess nutrients in your pond and boosting your water body’s aesthetics, but you should never rely on them as your main source of oxygen.
This is because, while they’ll do a great job of helping aerate during they day, your plants will actually consume air from the water overnight. This is why some pond owners who keep fish will find them at the surface in the morning; they’re not up there because they’re hungry, your fish are gasping for air. With that in mind, plants are still a valuable part of your aquatic ecosystem and can help ease the maintenance requirements of your pond so they’re definitely worth looking into.
If you’re looking to support your filtration system, reeds could be a great option for you. These guys are often used in regeneration zones when converting swimming pools to natural ponds because they help remove debris and other nasties from the water and they’ll perform the same function in your pond. They also help provide habitat for aquatic birds if that’s your thing but remember that this will increase the amount of maintenance required to keep your pond healthy.
If it’s cover for your fishy friends you’re after, however, Waterlilies or Water Hawthorn will better serve your purpose. Both plants are quite aesthetically pleasing and they’ll work together well as lilies tend to like warmer climates and Water Hawthorn will thrive in Winter, providing almost year round protection from ground and air predators.