Choosing Filtration Methods for a Koi Pond

Each fish produces plenty of waste in the form of ammonia, which is then broken down by bacteria and oxygen into nitrites and then subsequently broken down into nitrates. Overstocking fish or using underpowered filtration systems will lead to excessive nitrite and ammonia levels that damage your beautiful koi. It is essential to keep levels in check with routine water tests and the right filtration equipment. Koi need the following water quality rates for good health and rapid growth:

  • Ammonia – zero is ideal; .5-1ppm only for a short time
  • Nitrites – zero is ideal; less than .25ppm is acceptable
  • Nitrates – 20-60ppm is acceptable

Weekly or bi-monthly water changes go a long way in maintaining good water quality, along with annual or seasonal sludge removal and careful feeding habits. However, all ponds require filters to remove solid and chemical waste. There’s no way to keep full-sized, mature koi in a pond that doesn’t have at least a biological filter. For best results, you need more than one filter.

Mechanical vs Biological Filtration

For healthy, thriving fish, a filtration system specifically designed for koi ponds should be used. Even the smallest ponds, with just one to two fish, need filtration for your koi to be at their best. There are two types of water filtration: mechanical and biological. Each has a role in keeping your koi healthy and free from illness.

Mechanical filtration physically removes waste and solids, like sweeping up trash with a broom and dustpan. In water systems, however, solids are not the only dangers of waste. Even if the water appears clear, dangerous invisible substances can remain. Ammonia is highly toxic to fish, so a little extra food waste or a higher rate of fish waste can suddenly lead to unexpected die-offs unless you have a strong filter system. Sieve and rotating drum filters are two of the most popular systems in the mechanical category. Both feature fine mesh screens that remove particles and dissolved solids suspended in the water. Bead filters are also popular and offer more control in addition to serving as a type of biological filter.

Biological filtration uses helpful bacteria to detoxify the water and remove chemical components like ammonia. These bacteria need a specific, porous surface to grow on so there is plenty of surface area for the tiny lifeforms to colonize. You don’t clean these filters with harsh chemicals because that would kill the bacterial colonies doing the hard work of cleaning the pond water. Instead, you follow a procedure called backwashing in which you use water you drained during a water change to clean debris off of the filter media. Biological filters tend to rely on mats of synthetic felt or fleece to harbor the proper bacteria or may take the form of baskets filled with expanded rock balls.

Water Pumps, Drains, and Plumbing

You’ll need to move as much water as possible through the filters each hour to maintain high water quality. This means you’ll need bottom drains that can take in at least 2000 gallons per hour, with the appropriate number for the total volume of the pond. These drains need to connect to quality pond pumps to cycle the water through the filters and back out the tangential pond returns (TPRs). This creates an enclosed system that maintains high water quality while minimizing your monthly water bills for topping off the pond.

Sizing the Filters

When purchasing filters, it is recommended to double the size of the filter for your pond. For example, if your pond is 1000 gallons, use a filter made to handle 2000 gallons as a safety buffer.

Air Pumps

Most koi pond owners also add an air pump in the filtration system to increase total dissolved oxygen before water is returned through the TPRs. This is in addition to air stones and diffusers added throughout the pond to supply a steady amount of oxygen. Filter air pumps tend to be large water tumblers that use basic agitation to introduce a steady stream of bubbles. Air stones are rarely used in filtration units since they can clog from exposure to debris trapped in this part of the system.

UV Additions

If you want the advanced algae control offered by UV light treatment, you’ll need to add a bulb in the filter stage. You can’t expose your fish to the light on a regular basis and achieve good growth rates. The light prevents the algae from completing its life cycle, causing it to die off rapidly when it can’t reproduce to replace itself. Don’t be concerned that a UV lamp in your filtration unit will kill off the beneficial bacteria in your pond. Since only a small amount of the water is exposed to the light at any given time, there’s plenty of time for the beneficial colonies to repopulate each day. A UV bulb also won’t reduce the need for other types of filtration because it primarily kills algae when installed on a pond rather than reducing nitrates or killing off large amounts of bacteria.

Size your UV light based on its output wattage, which determines how much actual light is produced to kill algae. This often varies greatly from the input wattage that only tells you how much electricity the unit uses. Check the maximum flow rate of the light and match it to the flow of your filter system. For example, a filter system that has a flow rate of 850 gallons per hour (GPH) will work well with a UV light rated for 900 GPH.


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