Answer: If you plan to raise koi in your pond then you will probably need a filter in order to keep the koi fish healthy. (Koi require special water conditions.) If you plan on adding a few of the goldfish rather than fancy koi, then you may not need a filter.
The "secret" to a healthy pond is to maintain a balance between plantings and fish. The fish need oxygen in the water and this can be supplied by underwater plants called oxygenators such as Anacharis, or by the water fall/circulating water.
The plants also work as a filter and use up nutrients in the water. If you have too few plants, or too many nutrients, there will be problems. The fish waste supplies nutrients, as does excess fish food. If you overstock the pond with fish, or if you overfeed, they will foul the water and you will need a filter.
The other consideration is algae. If there are excess nutrients in the water, the algae will increase to undesirable levels. Using your underwater plants and covering the water surface with floating foliage plants such as waterlilies to prevent light from reaching the water will work together to prevent excessive algae.
When you first fill your pond, and again each spring, there will be algae. It takes a few weeks for the plants and water conditions to settle in and come into balance.
Keep in mind that a healthy balanced pond will not have totally clear water. You should be able to see your hand down about ten inches, but then it will be murky.
Finally, follow the manufacturers directions for pump maintenance and clean out. There are usually screens to protect the pump from debris that accumulates in the pond, and these need to be cleaned off periodically.
I hope this helps you plan your pond.
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