A good pond filter system is generally required to keep a garden pond or water garden healthy and clear. A Koi pond requires more filtration than a water garden or a goldfish pond due to the large mature size of these beautiful pond fish.Effective pond filters combine mechanical and biological pond filtration. Biological pond filtration involves beneficial bacteria to brake down fish waste and other organic matter. Mechanical pond filtration traps particles in some type of media for later removal during cleaning. Good pond filters accomplish both and can be fitted with Ultra Violet Clarifiers for algae control.