Install a stationary pond aerator or a floating aeration kit. There are a wide variety of pond aerators on the market ranging from around $200 to over $1,000. The motor of the aerator sits nearby the pond while a hose (either above or underground) pumps oxygen into the pond. The larger the pond the bigger and more powerful your aerator will need to be (and thus the more it will cost). A cheaper alternative is to buy a floating aeration kit. These work great for small and mid-sized ponds.
Install a decorative sprinkler in your pond. Depending on the size of your pond, you may need to install two or three sprinklers. Decorative sprinklers work similarly to aeration kits since they float on top of your pond. Water is brought up through the sprinkler and then redistributed into the pond creating air bubbles in the water. You can choose from a wide variety of sprinkler sizes, as well as sprinklers that feature lighting to illuminate your pond at night.
Install a waterfall mechanism and pump into your pond. Waterfall kits come in various shapes, sizes and prices and are made to look like natural waterfalls. You install a waterfall box at one end of your pond and a filer box and the opposite end then connect the two with an underground hose. A pump connected to the waterfall box pumps a steady flow of water through the system, redistributes the water into your pond through the waterfall, increasing the oxygen level.
Limit the number of fish or plants you have in your pond. The more you have the more oxygen you need in the pond. There are different kinds of pond plants, like water hyacinth and anacharis, that emit oxygen into your pond during daylight hours but then suck oxygen during nighttime, so be wary of such plants. Be vigilant about cleaning out dead leaves from your pond in the fall and early spring as the dead leaves will emit noxious gases that can suck oxygen out of your pond and kill your fish. Use a skimmer net to remove any leaves.