The Goldfish Pond, also relatively low-maintenance, needs aeration, water, plants and a small filtration system. The Koi Pond requires exellent water quality. The need for adequate and extention filtration is a must. When fish are involved, filtration is a pre-requisite. The amount of filtration depends on the quantity and size of the fish.
Plants do help clarify pond water and nitrifying bacteria does grow all over the pond.
Fish do 2 things. One, they give off ammonia through their gills and in their waste. Two, they eat all the time. When you put fish in a pond, you have created a cess pool. Food goes into your fish and sewage comes out.