Answer: If you raise koi in your pond you will probably need a filter as well as a pump to keep the water oxygenated. They are very demanding in terms of water quality. If you raise a limited number of ordinary or fancy goldfish you can do so without a filter (and also without a pump) if you do not overstock it with fish and also have enough plants growing in your pond. The goal is to create a sustainable balance.
You need surface plants to shade the water (your lilies for example should cover about 75% of the surface in midsummer), bog or edge plants to help use up nutrients, and also underwater plants called oxygenators to help keep the water clean and also use up the nutrients in the pond from the fish food (overfeeding fish is one of the biggest causes of algae and other water problems) and fish wastes. The oxygenators are usually sold in bundles and can be planted in pots and set at the bottom of the pond. A bundle per square foot of pond bottom is not too much. These also provide shelter for any baby fish.
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