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A biological filter uses bacteria to remove organic waste that can accumulate in your pond. You could get an in-pond filter or an external filter. In-pond filters have a pump attached to a fountain that runs into the pond. While they are easy to setup and install, they are difficult to clean because you must remove the entire contraption from the pond. External filters are very easy to clean, but it can be difficult to get the right size and pump, since they are usually designed for pools, not ponds. While these bacteria occur naturally in ponds, they are not present in the quantities necessary to adequately filter all the toxins that occur in most ponds. Get a filter capable of filtering about twice the amount of water that our pond contains. For instance, if a filter says it can adequately filter 3,000 liters, you should use it only if your pond is around 1,500 liters or less. If you get a filter that matches your pond's volume exactly, it will struggle to keep the pond clean. Your filter should filter your entire pond in about two hours. Check the speed of the pump and compare it to your pond's volume to choose the right one. As barley straw decomposes, it releases a chemical that prevents algae from growing. One bale of barley straw treats 1,000 gallons of water. In other words, if your pond holds 100 gallons, you'd need only one-tenth of a bale. The best way to add the straw is to wrap a softball-sized portion of straw in mesh netting and toss the balled-up straw in the pond. Alternatively, you could add liquid barley straw. One tablespoon usually treats about 50 gallons. If you don't notice a change after about a week, add more gradually. Barley straw is readily available at most garden centers. Barley straw is safe for use in ponds with fish. You could add both floating plants and submerged plants to your pond. Both will keep algae at bay and keep your pond clean in a natural way. Add floating plants. Lilies and lotus reduce the total surface area of the pond that algae can thrive on, and absorb nutrients that algae would otherwise use to grow. Submerged plants like hornwort, parrot's feather, and anacharis add oxygen to the water, preventing algae. They can be kept below the water's surface by attaching them to a rock or putting them in a plant pot and adding weight until it sinks.
Use a biological filter. Add barley straw to your pond. Add plants to the pond.