Deciding what type of filtration your new or existing pond needs depends on the level of water quality you wish to achieve. Do you want clear water for your fountain display? Or do you need to sustain the life of plants and koi for many years? At the very least, most pond keepers want clear, algae-free water. Koi enthusiasts demand more pristine water conditions with no ammonia or phosphate, and plenty of nitrifying bacteria to break down dangerous waste products into beneficial fertilizers.