The answer depends on the details of the pond. If you have a wildlife pond with no Fish, then the answer is no you do not NEED a filter system for the welfare of the Fish however it is something to consider for the point of view for water clarity as the water has a high chance of going green during the summer months.
If you do have Fish in the pond, whether that’s Goldfish, Koi, Sturgeon or any other Fish then yes you NEED a filter system. The reason for this is simple, without a working filter the organics in the pond; the Fish waste, left over food, sludge and silt, will break down into ammonia. This is a toxic parameter to Fish and builds in concentration the longer the organics break down. A filter acts as a breeding ground for beneficial bacteria to grow which breaks down this ammonia into less harmful chemicals which are later diluted through rainfall, top ups and water changes. Without an adequate filtration system your pond water will become murky and green, the water parameters will become increasingly toxic and ultimately will end in loss of Fish.
