Choosing the right filter for your koi pond is one of the most important decisions you can make. You need make sure it is properly sized, can handle the flow from your pump and will remove all the waste produced by your koi. It also needs to be easy to clean and provide enough surface area to convert all the ammonia out of the koi pond. It also needs to be well made, efficient and of course fit your budget.
There are two basic types of filters for koi ponds, gravity flow and pressurized. In general, most gravity flow filters are more efficient at removing sold waste and provide a better environment for growing beneficial bacteria. But pressurized filters are usually much easier to install and hide from view. Pressurized filters also have the added benefit of an effective backwash system.
The first thing you need to choose is your pump. For more on choosing pumps, please read our article on pond pumps. The pump flow will determine the minimum size of filter you will need for your koi pond. If water flows through the filter too quickly, then it simply will not be as effective. With gravity filters, in general, the bigger the filter is in comparison to the pump the more effective it will be because it utilizes dwell time (the length of time the water is in the filter) for both mechanical and biological filtration. With pressurized filters, you don't want the filter to be rated too much larger then the size of the pump. The reason for this is a small pump cannot effectively backwash a larger filter. If you do decide to oversize your filter in comparison to the pump, then you may want to consider a blower to help backwash the filter. This is not to say you shouldn't oversize a pressure filter in comparison to the size of the pond, this is usually a good idea. But rather, if you increase the size of the pressurized filter, then generally the pump needs to be increased as well.
So, how do you size a pressure filter? Well in general cut the manufacturers rated number of gallons in half. So if the filter is designed for a 3000 gallon koi pond, use it for a 1500 gallon pond. If you have a 3000 koi gallon pond, get a filter designed for a 6000 gallon pond. It may seem like you are greatly oversizing the filters, but really it's the proper size. The reason for this is most pressurized filters are marketed as a stand alone filter. That means it will be used as both a mechanical filter and a biological filter in one. So unless you want to backwash your filter every day, get one designed for double the size koi pond, that way the waste won't muck up the media nearly as quickly.
Now if you have a separate mechanical filter prior to the pressurized bio filter, then you might be able to get away with a smaller unit because the bio filter will receive clean water and will not get choked out with muck. Having two separate filters, one for mechanical and one for biological is the best option and will provide the best quality water for your koi and be the least amount of maintenance.
So now we can talk a bit about gravity fed systems. These are systems were the pump is at the end of the filtration pulling water through, rather then forcing water into a pressurized system. Gravity fed systems are in ground along side the pond. Water flows from the koi pond and into the filter. The pump is on the other end pulling water and sending it back. Just like a pressurized system, its best to have two separate filters, one for mechanical and one for biological, however there are systems on the market the offer both in one. In general the gravity fed system will be more effective at removing solid waste. The reason is the koi waste remains in tact and is not blended up by the pump. The waste gently flows into the filter and is easily removed.
Some of the best koi pond systems I have seen use both gravity and pressurized systems. That way your koi get the best of both worlds. The gravity system is first, removing solid waste and perhaps providing some biological filtration. Then the pump pulls from this systems and send it into a pressurized system that gives additional biological filtration and removal of fines. This of course is a more expensive option and it requires more space for your filters, but for the serious koi ponds, its going to be the best possible method. A good example of this excellent set up would be a Nexus filter set up for gravity flow, with a pump pulling from the nexus and sending it to an pressurized filter bead style filter like the Ultima or Alpha One before returning to the koi pond.
Whatever you choose for your koi pond, be sure the system is easy to clean, provides effective removal of solids before the biological filter and has enough surface area to handle all the ammonia produced by the koi.