There are many different ways in which koi ponds can be filtered. There is often a lot of confusion surrounding filtration, and many new koi pond owners get confused and overwhelmed by the different types of filtration available. Filtration is actually very simple but there are different ways in which it can be done. You can have a separate mechanical and biological system, which is best for larger ponds and then for smaller ponds you get mechanical and biological systems in one.
The most common problem in smaller ponds is the misunderstanding that the sponge inside a submersible water pump is good enough to filter their water. This is not true. This sponge is designed to filter out dirt that could damage the impeller inside the pump. If dirt or even stones get inside the pump it can jam the impeller or even break it. When you notice your water pump is pushing out less water, you must remember to switch the pump off and open the front of the pump to get to the sponge and clean it or replace it. This will make sure that you get the correct flow rate from your pump at all times. There are small filters designed for small ponds, which are available at your koi stores.
Filtration consists of two components, the first is mechanical filtration. Mechanical filtration is the removal of solids such as dirt, algae and organic matter. The second component is the biological filtration; this is where the dissolved toxic excretions of koi are filtered out e.g. Ammonia is converted into Nitrite and then into Nitrate inside the bio filter. This is done by a group of bacteria called nitrosomonas bacteria. When buying filtration for your pond, you must have both mechanical as well as biological filtration. If space is a problem around your pond you will need to use a pressure system. You can use an external pump that sucks water from your bottom drains into a settlement chamber into a koi sand filter that will then run into biological bio tubes and an ultra violet sterilizer. The koi sand filter and settlement chamber are your mechanical filtration, where your solid dirt is trapped. This must be maintained and a weekly hand backwash is required on your koi sand filter. The bio tubes will be your biological filtration where your good bacteria will grow and convert your toxic wastes from your koi into a less harmful substance called Nitrate. To get rid of nitrate you will need to change some of your old water and replace it with tap water that has being decholrinated. If space is not a problem you can use large filter boxes as your mechanical and biological filter. Your mechanical filtration can be the sponges on the top of the box, or else an extra chamber built inside the same filter box. Inside this extra section you can place filter brushers to trap your solids. If your koi pond is large you would use a combination of a pressure and gravity system. It is far safer this way as if one pump stops working you will always have another pump running the other system.
The filter takes time to mature with bacteria, so only add a few koi fish in the beginning until your system is at least 6weeks old. If you want to know if your filter is working, you need to test your water and when your ammonia and nitrite levels are at the lowest levels, your filter will be working.