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Koi Pond Filtration

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To achieve an efficient biological surface we must ensure that the water that enters the filtration is rich in oxygen. This helps the friendly bacteria to oxidise the waste products through the nitrogen cycle. This is called aerobic filtration. Filter systems must fulfil the following criteria:

  • The filter volume should be at minimal 10% of the ponds volume.
  • The flow rate you should aim to achieve is to turn over at least half the ponds volume once per hour.
  • The filter should have several chambers which can be which can be kept separate and emptied individually when cleaning. And a vortex should be included.
  • The biological surface area should be at least one square metre per ten cubic metres of pond water.
  • An ultra violet clarifier should be installed to eliminate single celled algae, which causes green water.

Bottom Drains

In our experience a bottom drain is the best method of feeding a filtration system. Bottom drains efficiently remove dirt and debris that settles on the pond floor. These drains can be easily installed and at a relatively low cost. They can be incorporated in a variety of types of pond construction. They can be used with flexible rubber and butyl liners, rigid plastic liners, concrete liners and block built rendered ponds. There is a little bit of extra effort when installing one of these drains as it is important to set the drain in a concrete block for extra stability below the pond.

Filtration Types

There are two main types of filtration; gravity feed where the water is removed from the pond via a bottom drain. It is then forced via gravity through the filter which is sited buried in the ground close to your pond so that the water level in both your filtration and pond are the same. The water is then returned to the pond via a dry sited pond pump through an outlet in the side wall of the pond below the waterline. At this stage it is important to use a venturi on the outlet to oxygenate the returned water as the biological surfaces will have depleted the oxygen during oxidisation of the waste products. The second main type and probably the more familiar type is a pressurised feed filter. This involves an internal solid handling pump at the deepest part of the pond pumping water through a flexible pipe up to the filter which is sited above the surface level of the pond, which allows the water to return by the forces of gravity.

Filter Process

Your aerobic filter should cover the two main stages of filtration. The first stage is mechanical filtration, this is where all suspended debris and dirt should be removed and trapped via mechanical filter media. The vortex is a mechanical part of the filtration, and should be located as the first chamber in your filtration. In the vortex water is swirled in either an upward or downward movement through a cylindrical chamber. As the water swirls upward or downward heavier suspended debris sinks to the bottom and can be emptied through a gate valve at the bottom of the chamber. In the next chambers of mechanical filtration you could use a variety of media including floss, sponges and brushes the later being the favoured method.

The second stage of aerobic filtration is the biological surface. For a biological surface to work efficiently the water passing through it should be oxygen rich and free from dirt and debris. The biological surface area should be at least one square metre per ten cubic metres of pond water. Medias that are suitable for the biological surface are sponges, porous lava stones, flow core (pieces of chopped up corrugated pipe) and Japanese matting the later being the favoured method. Please note biological filter media should never be washed under a tap or hose as tap water contains chlorine which kills off friendly bacteria. Once the bacteria is killed off it could take up to ten weeks to reach maturity again and in a well stocked pond toxin levels can spike during this period. Fish loss and disease under these conditions is inevitable. If biological media has perished and needs replacing, replace 50% initially then eight to ten weeks later replace the other 50%. During this change over period check water quality at regular intervals.

The by-product of the aerobic filtration through the nitrogen cycle is nitrate. Nitrate is considered relatively less toxic compared to ammonia and nitrite but in high quantities can still cause stress and discomfort to your fish. Nitrate levels can be reduced using various methods. A protein skimmer removes proteins from your pond before they are broken down therefore less of the toxic by-products are produced. Plants are also a means of reducing nitrate levels, so the introduction of a well planted bog garden post mechanical/biological stage with a rapid throughput would be very beneficial. Please take care with the latter as this will increase pathogens in your pond, use an ultra violet sterilizer to kill off these pathogens in the water returned from your pond. Watercress is ideal for a bog garden as they remove large amounts of nitrate, but be sure to keep them trimmed back to stop them flowering and also to encourage vigorous new growth ensuring optimal nitrate removal. In our experience nitrate removal is not that essential but some may disagree.

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