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Clean, well oxygenated quality water is the key to a successful Koi or goldfish pond.  A good filtration system is an integral part of any pond, but is sometimes overlooked by the inexperienced homeowner or landscaper. You want something that is pretty to look at and enjoy.  Koi ponds are closed recirculating water systems.  Fish waste and other debris can build up and turn the water toxic unless your water is properly filtered, oxygenated (waterfalls are great for this), and flowing (stagnant water results in carbon dioxide build up).  Plants are a great natural filtration system and algae inhibitor. Netting placed over the pond in the fall keeps the leaves out.

Experience has taught us the following elements keep a pond clean:

A Pond Skimmer

A pond skimmer is a box with an opening at the pond’s surface.  The box houses the pumps which feed the waterfall.  Water is pumped out of the skimmer box to the biofalls filter.  The skimmer continuously removes and collects debris such as leaves floating on the pond’s surface. A net or basket in the skimmer box collects the debris before the debris can clog your pump.  A foam pad may be placed in the skimmer to collect finer particles.  The pad and basket need to be periodically cleaned to prevent solid matter buildup which will block the water flow to the pump.  A pump inside the pond is unsightly and will become clogged with debris, reducing water flow and eventually causing the pump to fail prematurely.  The skimmer collects water at the surface only.  If a leak occurs in your pond, the pump cannot pump your pond dry, leaving your fish without water.  The skimmer uses the highest oxygen surface pond water, which is essential for maximum productivity of your bio-filter.  In winter, the skimmer only recirculates the top water, reducing heat loss.  A good skimmer for ponds with pumps under 7500 gph is manufactured by Savio  http://www.savio.cc/pond-products/C10/

A Biofalls Filter

The biofalls filter uses foam pads for mechanical filtration and a less dense medium for bacteria colonization or bio filtration.  The foam pads screen debris particles in the water.  Varying mesh sizes may be used.  Foam pads need to be cleaned periodically by removing them and hosing them down.  Bouncing the pads against a fence or wall will remove much of the debris.  A pressure washer simplifies this process. If the water used to clean the pads contains chlorine you may want to soak the pads in pond water before reinserting the pads into the filter.  The chlorine may kill the bacteria in your Biofilter.

The Biofilter medium (plastic mats, leather strips in a netted bag, or other products available at your local pond store) is used to support large colonies of bacteria and enzymes to filter the water of fish waste and organic debris.  The biofalls also aerates the water.  Oxygenated water supports more fish and plants, helping to create a balanced ecosystem.  The more fish you wish to have, the larger the bio-filtration area needs to be to eliminate ammonia build-up.  Plants can be added to the top of the biofalls to increase its filtration capabilities.  Biofalls plants include Water Hyacinth and Water Lettuce.  Anacharis, an underwater plant, is a great oxygenator.  Aquascape is a maker of the Biofalls filter.

Bog filters

A bog pond separate from the main pond, often contained in the waterfalls flow area, and also called a veggie filter, can help purify the water.  Gravel in the bog pond gives the plants a place to root and bacteria a place to colonize.  Water must constantly flow through the bog pond.  Some bog ponds place the water flow in a perforated pipe beneath the gravel.  The filtered water from the biofalls will be rich in nitrates.  A bed of greedy aquatic plants will consume a great deal of these nitrates before the water flows back into the pond.

This Veggie Filter is an intermediate pond located between the biofalls filter above and a well stocked Koi pond below the waterfalls.  Anachraris and water lillies are used as the main filtering plants.


The anachrais in the above veggie filter is shown here with colonized bacteria.  The anachraris helps consume the nitrates from the above biofalls filter.

Bottom Drain

A bottom drain may be placed at the bottom of the pond with the plumbing running underneath the pond to an external pump.  You may consider setting the bottom drain in a concrete base with the bottom drain encased in it.  Any mistake in installing the bottom drain will be very hazardous and possibly uncorrectable.  You may have to totally disassemble the pond to fix the problem.

Oxygen Control

The amount of oxygen dissolving in water is temperature dependent.  Warmer water contains less oxygen than colder water.  Constant water flow is imperative in a Koi pond.  The more fish a pond has, the more important this is.

Since warm water can hold less oxygen than cold water, the summer months are the most vulnerable period for low oxygen levels, and the level at night is the most critical of all.  During the day, water plants and algae release oxygen into the water during the process of photosynthesis.  At night, this cycle is reversed, with plant and animal respiration using up oxygen and producing carbon dioxide.  It is not surprising, therefore, that warm summer nights can result in dangerously levels of low oxygen.  Warm summer nights and low atmospheric pressure on stormy nights can result in dangerously low levels of dissolved oxygen.

This is why it is important to run your pumps 24 hours per day, 365 days per year.  The oxygen they provide to your fish is their lifeblood.  Don’t turn your pumps off at night!  The healthy bacteria in your filtration system also require constant flow.  Turn off the pumps, and these bacteria which stabilize the water quality will soon die, and create a very foul odor when the pumps are turned back on.

A venturi is sometimes used to increase oxygen levels in the water.  A venturi tube allows air bubbles to be inserted into flowing water and released in a tube near the bottom of the pond.

Ultra Violet Light

UV sterilization is a proven method for controlling waterborne algae. The wattage of the U.V. light and the flow rate of the water through the light must be properly matched to your pond for maximum performance.  If the water flows too fast, the ultraviolet light will not have time to kill the algae.  Using a U.V. light to kill microorganisms and pathogens requires even slower flow rates.  Combining sterilization with adequate mechanical filtration, and operating the two properly is most effective in eliminating algae blooms and maintaining clear water. This combination will not, however, control nitrogen or carbon dioxide levels. Practical fish stocking densities and responsible feeding, along with routine filter and UV sterilizer maintenance play a big part in achieving a balanced system. Partial water changes (approximately ten percent of the pond volume weekly with non-chlorinated water) will aid in diluting nutrients. Artistic Landscapes recommends ultra violet lights by www.emperoraquatics.com

The Nitrogen Cycle

Ammonia and nitrogen build-up is the biggest threat to your fish health and water quality. Nitrogen in the system, commonly referred to as Total Ammonia-Nitrogen (TAN), is a by-product of protein metabolism.   The gills of the fish produce toxic ammonia as they digest food.  Fish waste products, uneaten fish food, plant fragments and dead animals also produce ammonia when they are consumed by bacteria.  When the water in a system has a pH of about 7.0, the system is stable and the majority of TAN is ionized and does not adversely affect the system. But when a system has a pH of about 9.0, up to 30 percent of TAN may not be ionized and levels of ammonia-nitrogen can become extremely toxic to fish.  Neutralizing these harmful toxins requires the build-up of beneficial bacteria in the pond and most importantly your filtration system.

There are two stages in the breakdown of ammonia in a biological filter system, each stage involving different types of bacteria to turn the harmful ammonia into nitrites and nitrates.  In the first stage, ammonia is broken down to nitrite by a number of different nitrifying bacteria.  A second group of nitrifying bacteria, principally Nitrobacter, converts the nitrite to nitrate.

Both groups of these bacteria require oxygen to thrive and purify the water.  The nitrifying bacteria require an oxygen level of at least 1mg/litre in the water continuously flowing through the filtering medium,  Sediment in the bottom of the filter can deplete oxygen levels and encourage the growth of anaerobic bacteria, rendering the filter ineffective.  It is important to keep sediment to a minimum in the filter.

The biological filter chamber in the bio-falls filter contains a filter medium with large areas for the nitrifying bacteria to grow.

Installing a biological filter system to a pond does not guarantee an immediate improvement to the water quality.  A new filter must develop beneficial bacteria to filter the water.  This takes time.

Ammonia and nitrite levels are likely to fluctuate in a new filter and it may take from six months to a year before the filter has finally matured.  The concentration of ammonia may increase dramatically, sometimes to dangerous levels, once you introduce Koi into a new pond.  We call this the new pond syndrome.

The nitrification process develops much faster in warmer water, much slower if at all in the winter (below 41 degrees).  The nitrification process can be sped up by adding live cultures of nitrifying bacteria such as Microbelift.  See www.microbelift.com for more information.

Nitrifying bacteria are the foundation of biological filtration and safely grow on all surfaces of the aquatic environment, including substrate, plumbing and the walls of the tank.

Carbon dioxide can also accumulate in re-circulating systems as a by-product of fish and bacteria respiration. A well-designed re-circulating system should maintain adequate dissolved oxygen levels while minimizing carbon dioxide concentrations.  Carbon dioxide, harmful to fish, can build up in the water, especially at night. "De-gassing" is a term, most commonly used within the aquaculture industry, to best describe stripping carbon dioxide from water. De-gassing is a simple procedure that involves aerating water that is low in oxygen, removing carbon dioxide, and at the same time, increasing dissolved oxygen levels. Packed media columns, venturi air injectors or moving-bed bio filters can accomplish the same results.  Flowing waterfalls are the best oxygenator.

The better your filtration and water flow system, the better the water quality will be.  The fish stocking density can increase, and providing great water quality will provide your Koi a happy home!

Ponds Introduction    Pond Design    New Pond Setup Filtration    Algae Control    Lighting  Your Pond
Water Plants    Pond Fish
Return to Landscape 101