First you must understand that there is Filtration and then there are Filters. The Filtration is basically the type of filtering a system performs, such as Mechanical, Chemical or Biological. The Filter is the actual unit that performs the job. Let’s first discuss the different types of Filtration.
Mechanical Filtration – refers to the removal of large and small particles through a material that traps the particles. This is a very important part of any filtration media. Mechanical Filtration helps clean the pond water of solid wastes like uneaten food, dry leaves, feces and plant debris before it reaches the bottom of the pond or aquarium, thus keeping the water free of decaying substances which can later produce harmful chemicals. Mechanical filters sometimes come with skimmers to catch large floating debris; this is followed by a filter which catches smaller debris and prevents it from entering the pump. Therefore, a mechanical filter can be considered the most important part of pond or aquarium filtration.
Chemical Filtration – refers to using chemicals to remove chlorine, pesticides, heavy metals, ammonia and other chemical impurities from the pond or aquarium water by chemically binding them to the filter media such as activated carbon.
Biological Filtration – refers to a filter that provides a place for Beneficial Bacteria to grow, which helps remove pollutants from the water. You will have to add material such as matting or gravel for the beneficial bacteria to live on which is one more item to maintain. Also, if you use the wrong type of water, such as city water to clean the biological filter you can kill the beneficial bacteria. A biological filter will take weeks or months to mature enough to provide any benefit to your pond or aquarium.
Now let us discuss the type of Filters
Sand Filters – while sometimes highly recommended for ponds they should in fact only be used for swimming pools. They are not capable of trapping all the waste in the sand, thus allowing un-trapped particles to re-enter the water. They are extremely hard to clean as they must be back-washed on a daily basis which causes a significant waste of water and they must run 24 hours per day for the best filtration. Also if you have hard water, it can lead to the sand clumping at the bottom of the tank reducing filtration.
Bio Bead Filters – these are very similar to the sand filter, except that they are even harder to clean.
Submersible Filters – are hard to maintain as you may need to wade into the pond to retrieve the filter for cleaning. The waste from the filter may spill back into the pond during the cleaning.
Cartridge Filters (Swimming Pool Filters) – While most people balk at the idea of using a swimming pool filter for their Koi pond, we have found them to be the best all around filters. They filter out 99% of the waste, are very easy to clean and last a very long time. You can also use these for aquariums.