Not having enough oxygen in a pond can lead to sick or dying fish and plants. Some plants actually help to reoxygenate a pond, such as aquatic plants. Watch for warning signs like decaying plants at the pond's edge, strong odors, algae overgrowths, and gasping fish to know when your pond needs action to oxygenate it. You also can test your pond before the danger point by using commercial oxygen tests on a yearly basis. The steps below may be stopped at any point when your pond is responding to oxygenation, or you can continue for further actions.
The best thing to try to increase oxygen quickly in a pond is to install an aeration device like a bubbler or fountain. This mixes oxygen in with the water, and keeps the water moving so it is not stagnant. They come in many forms, including airdomes, air stones, pumps, and diffusers.
For longer-term aeration, build a waterfall into your pond. Create a slight hill on one shore of the pond with fill dirt, and line the slope with pond liner and rocks to guide the waterfall into the pond. Breaking up the fall of the water with flat shelf-like rocks is both attractive and oxygenating. You will need a water pump or waterfall plumbing kit to keep the water circulating back up to the top of the waterfall as well.
Add a good mix of oxygenating water plants, like pond lilies, marginal plants and aerobic plants. This will keep down bothersome algae, provide food for pond fish and animals, and help add oxygen to the pond. Floating plants also have the benefit of providing more water movement when combined with aeration devices. Some good choices include anacharis and cabomba, both of which help add oxygen to the water. Others are tapegrass, ludwigia, hygrophilia, vallisneria, moneywort, water hornwort, and sagittaria, depending on your climate and existing plants.
Keep pond filters clean. Change them yearly, and clean them regularly throughout the year to be sure they are not blocked up with muck or waste. Be sure to clean them before leaving for an extended period of time especially.
For winter maintenance, keep the oxygen level in your pond healthy by installing an aeration kit or a de-icer kit. This will keep a small hole open in the ice, so that oxygen can be exchanged from the air.