There are many different commercially available Pond filters.  Each with its advantages and disadvantages, there is a use for most of these Filters.  Submersible filters are enclosed devices that sit inside the Pond.  Usually used in conjunction with a fountain, they are capable of providing adequate mechanical and chemical filtration to smaller ponds.  External filters come in a wide array of shapes and sizes.  They can be any size, and will sit outside from the Koi Pond, with water piped from the pond and back into it.  Although somewhat more complicated, external filters provide much greater flexibility when it comes to filtration systems.

Submersible Filters are a simple way to add filtration to small ponds.  They are self-contained devices that often return water through decorative fountains, making them an aesthetically pleasing addition to the Koi Pond.  The fountain attachment will usually obscure the view of the cords and box itself, making it a welcome addition to the submersible filter.  The drawbacks of submersible filters are generally maintenance ones.  To perform routine maintenance, the filter must be dragged up from the bottom of the pond, making this an impractical option for larger ponds.  They also lack the flexibility of external models. To incorporate Ultraviolet Sterilizers, a submersible filter will need to pump water outside of the pond, or incorporate a submersible sterilizer.  Submersible sterilizers are generally more expensive, and bulky, making them difficult to hide inside a smaller fish pond. 

External filters are the more useful of the two styles.  The can be easily hidden from view, maintaining the natural look most pond owners enjoy.  The can be easily accessed for routine maintenance, making it less of a hassle, and as such, more likely to performed on a regular basis.  They are very flexible, with a variety of components that can easily be incorporated into their design.  They can also be as expensive or inexpensive as the pond owner would like.  External Filters can be fabricated from inexpensive parts that can be assembled into a very effective filtration device.  The main drawback to external filtration is their size and difficulty to add to a small system.  They must be piped in with a pump, which can be as easy or difficult as the pond owner makes it.  They also require some sort of shelter, or they will appear as an eyesore outside of the pond.