Koi Pond Filtration Basics

Koi Pond Filtration Basics


Successful Koi keeping revolves around maintaining an artificial environment that simulates as closely as possible the natural environment in which Koi are to be foundThis is achieved by using filtration to create a natural biological filtration system that keeps your fish healthy and high quality of water in the pond.  Water quality is intricately and intimately linked to the health and the well being of Koi. In a natural pond, the constraints that face Koi keepers are nowhere to be seen. Koi have literally millions of liters of water that surround them, and they simply cannot produce enough waste material to in any way significantly affect the vastness of the lake or dam in which they reside.  A Koi pond, by comparison, has a much, much higher fish stocking density. The wastes that Koi produce become extremely significant since they are not diluted by nearly the same volume of water as in a natural environment.  Good water quality does NOT mean water clarity (think about any dam or lake). However, it is possible to have good water quality AND good water clarity.  There are many different ways to filter pond water, and every 2nd man will have a different opinion on what the best way is. The methods discussed here have been tried and tested over 25 years of Water Plant being in the industry, and when applied correctly are proven to work.

Filter Types

There are two main types of pond filter systems:

  • Gravity Filter Systems:

In a gravity pond filter, the water is pumped into the filter where it will pass through various layers or chambers and return to the pond via gravity.  Gravity Filters need to be positioned above the water level of the pond so that the water can travel back to the pond.

  • Pressurized Filter Systems: 

Pressurized Filter systems work very much like a pool filter system.  A pool pump pumps the water through a Sand Filter (with either larger sized pool sand or Zeolite), then through a biological filter chamber and a UV pond sterilizer. As the system is pressurized the filter can be lower than the pond and can pump the water up through a water feature. The pump must not be positioned higher than 1 meter above the pond level.

There are 3 types of filtration processes that take place in a pond:

  • MechanicalMechanical filtration is the removal of solid particles from the pond. It is important that Mechanical filtration is the first step in a filtration system as solids affect the efficiency of biological filters.  In Gravity filter systems, the media used for mechanical filtration is usually sponges or brushes. These will be placed on top of or before the biological filter medium. The media needs to be accessible, as it needs to be cleaned regularly as to not clog the system and cause the filter to flood.  In Pressurized Filter Systems the water will pass through a sand filter. A larger grain pool sand (4  – 13mm) is used, as there is far more debris in a pond and the filter will block much quicker.  Zeolite can also be used, this has the added benefit of helping neutralize ammonia from the pond which will aid in the biological filtration.The filter is back washed like a pool, but a manual backwash will need to be performed regularly to loosen up any compacted debris or media in the filter.  There are specially designed pond sand filters available with easy access lids and larger filter fingers that make maintenance of the filter easier.  
  • BiologicalBiological Filtration is the most essential components of an efficient re-circulating system in a koi pond and is required to remove Ammonia, Nitrite, and Nitrate. Ammonia is a highly toxic gas produced from organic wastes (leaves, dead algae, insects, etc) and as a metabolic waste produced from protein eaten by the Koi. In a natural environment, such as dams and rivers, it would be immediately diluted to harmless levels. However, in the confines of ponds, levels can rapidly rise to dangerous levels unless it is constantly removed, Millions of colonies of Aerobic (oxygen loving) bacteria grow in our Biofilters, and thus it is essential that the pump providing water to the biofilter is continuously running 24 hours a day. These bacteria perform a function called Nitrification (the nitrogen cycle), witch is the process of converting Ammonia to nitrites and then nitrites to nitrates. There two bacterial species involved. Nitrosomonas sp. bacteria which oxidize ammonia to nitrite, while Nitrobacter bacteria convert nitrite to the less harmful nitrate.  Plants and the “good’ algae that grow on the pond help remove the nitrates. These bacteria are naturally occurring after 6 to 8 weeks or can be boosted by adding a starter bacteria to the pond.Biological Filter MediaThe purpose of filter media is for the bacteria to grow on.  Ideal biofilter material will have a large surface area so that more bacteria can grow.  “Bio blocks”, alfagrog and plastic shavings are very good as they have a large surface area and can accommodate more bacteria. This media can be used in both pressurized and gravity filter systems.
  • UV filtrationA U.V (Ultra Violet) light installed on a pond acts as a clarifier and keeps your pond water clear and helps control algae growth. How it works. A U.V Sterilizer will maintain a clear pond without the use of chemicals. The unit works by exposing the phototrophic micro-organisms (free floating single-celled algae) to a dose of U.V. radiation. This destroys and prevents the re-growth of the algae. There are various sizes of U.V lights and are dependant on the size of the pond and require a specific flow rate through them to operate correctly.  The Ultraviolet tube weakens due to usage, and thus must be changed regularly. Even though it may still glow, the output of the tube cannot be determined by the naked eye. So regular replacement is the only way to ensure “peak performance”. Thus the tube in the pond U.V. units can be replaced every 12 months (approx. 8000hrs)

    Selecting the correct Filter for your Pond 

    Now that you have an understanding of the basics of filtration it is time to look at the factors that determine what filter will best suit your needs.

    • Pond Size

     The main factor when selecting a pond filter is the size for your pond. All our filter systems are design for specific pond sizes and a filter system that is too small will not effectively filter the pond. The equation for calculating the pond size is m3 x 1000. So if you have a pond of 2m x 3m x 1m deep the calculation would be:

    2 x 3 x 1 = 6m3 x 1000 = 6000 litres

    For ponds that are not square or different shapes, you can break them up into different parts and ad the total together. You may over calculate the size slightly but it is always better to overestimate than under. It is also important to factor in the circulation that is required for a pond. The rule of thumb is to circulate the pond volume every 2 hours. So if you have a pond of 6000 liters, you will need a pump that will circulate a minimum of 3000 liters per hour. The more circulation the better.

    • Fish Stocking

     The number of Fish you stock in your pond play a crucial role in the size of your filter. We recommend 1 fish per 1000l of water, as the fish will grow and a fish of 80cm will produce a lot of waste that the filter will need to process. We understand that most people will not stick to this so it is important to adjust your filter size accordingly. So if you have a 6000l pond with 10 large Koi, it is advisable to have a filter system that will handle 10 000 liters of water.  Once you have all this information you can effectively select the correct filter for you. Biofilters and UV lights are rated or specific sizes of a pond, and should state the maximum flow rates for them to help you in purchasing the correct units.

    Other ways of improving water quality

    • Venturies

     Water without oxygen cannot support fish or other aquatic life. For optimum fish keeping conditions and in order for fish to thrive there should be an oxygen concentration of at least 8 mg/l
    in your koi pond. The exchange at the water’s surface of carbon dioxide to oxygen is not sufficient, thus the use of a Venturi on the return line to the pond will result in better aeration and the water quality will be greatly improved.

    • Regular Water changes

    It is advisable to change 10% of your pond water on a weekly basis. If you are adding municipal water it is advisable to add a chlorine remover such as chlorine.

    • Regular removal of solids and sludge

    It is advisable to regularly remove any build up of debris and sludge at the bottom of the pond. This can be done by flushing debris through the bottom drain or by using a pond vacuum.