The Importance of Water Circulation

Submitted by Chris Neaves on Mon, 01/29/2018 - 17:09

The greatest effect of water circulation is that it prevents thermal and chemical stratification.” (Boyd)

Water circulation within the pond plays a vital part in successful Koi keeping.

The internal movement of water in a koi pond is an essential design feature. Pond water should never be stagnant. The movement of the water dilutes the impurities and distributes the dissolved oxygen throughout the system.

In our Koi ponds water circulation is critical for moving solid and chemical pollutants to the filters and biofilters. The water should circulate gently around the pond as well as vertically – either from top to bottom or bottom to top. The internal circulation does not have to be at a velocity that will over exercise and exhaust the Koi.

Circulating pond water:

  • Ponds will have concentrations of ammonia and low oxygen levels in areas where the water is still.
  • Circulating or moving pond water establishes a uniform dissolved oxygen profile within the water body.
  • The total dissolved oxygen content of the pond can be increased with circulation.
  • Ammonia is diluted throughout the pond volume.
  • Solids can be directed to drains and removed helping to clear the water and keep the organic load down.

To get a concept of how the water should be circulated within a pond, look at a tropical fish or marine fish aquarium through the side and see how the water moves and circulates.

Effects of Incorrect Circulation

An outlet directly opposite the inlet will create dead areas in the corners. By directing the inlet water along one of the walls of the pond a circular motion can easily be created.

An inlet at the surface and an outlet at the surface will create a large body of stagnant water at the bottom of the pond as well as temperature stratification.

Oxygenation and Circulation

The only way oxygen can dissolve into water is through contact with the atmosphere. In Koi ponds this can be achieved using different methods.

It is logical that the water be drawn from the bottom of the pond via bottoms drains or bottom suction pipe, pumped through the filters or drawn through the filters, then returned to the pond via a water fall or stream. Waterfalls are the most dramatic feature of the Koi pond other than the Koi themselves.

If the design dictates, the water can also be pumped into the bottom of a pond and the water can be pushed up-wards and overflow to another pond or overflow chamber or drawn from the top.

The breaking up of the water molecules by waterfalls or within the media bed of trickle filters assists with the exchange of gases in the water. Air stones can be used to create vertical movement in the pond as well. The breaking of the air bubbles at the surface and the vertical movement of the water caused by the air lifting the water towards the surface will add to the interface with the atmosphere.

Generating Circulation

Circulation, (moving the water around the pond in a circular motion), can be created by directing the returns from pumps, filter chambers or even by directing the water from a water fall or stream to create circulation.

Removing water from the floor of the pond via gravity fed pipes or by suction to the pump generates vertical movement of the pond water.

By-Passing Filters

By-passing filters, for any reason, will present problems of its own and should be avoided.

It is very important that all the pond water is fed through the filter system at all times.

The "dilution factor" can be significant. Filters that only process a portion of the water circulated because a portion of the water circulated bypasses the filtration system need a re-think in terms of design. The pump may be too large for the filtration system. This is especially true when purchasing preformed filters. Make sure the pump output matches the manufacturers recommended flow rate through the filter. You must also make sure the flow rate is appropriate for the pond volume. Water that is never truly clear, or ponds where the ammonia levels are continually high, is often the result of polluted water returning to the pond without being filtered.  Bypassing the filtration system with a portion of the circulated water reduces the efficiency of the filtration system.

Stagnant Water

Stagnant water or stagnant areas in a pond pose a great health risk for our koi. Stagnant areas are formed when there is a lack of circulation in an area of the pond or filter system. The lack of circulation results in organic sludge build-up and oxygen depletion in that area.

As the solids and organics build up, layers will be formed. The deeper layers will be cut off from the oxygen available in the passing water. Anaerobic bacteria grow in the absence of oxygen and will multiply in these areas of sediment.

Anaerobic bacteria produce a highly toxic compound called hydrogen sulphide. Hydrogen sulphide (H2S) is measured in parts per billion and is toxic to Koi in minute amounts.

Hydrogen sulfide is affected by pH, temperature, and dissolved oxygen. A pH below 7 will increase the toxicity of hydrogen sulphide. High temperatures will also increase hydrogen sulphide toxicity. High levels of dissolved oxygen will prevent the escape of hydrogen sulphide from the sludge unless it is disturbed.

Low levels of hydrogen sulphide can be relatively harmless in winter but can become very toxic in the summer months.

Large amounts of hydrogen sulphide can be released when sediments are disturbed on the bottom of ponds, in areas of the filters or during plant maintenance.

Hydrogen sulphide is considered to be as toxic as Cyanide. Low levels of hydrogen sulphide will produce high stress levels which can cause organ failure and slow deaths. Symptoms of low levels of H2S (hydrogen sulphide) poisoning are gill damage, skin problems, poor feeding and fish becoming thin. Fish also become more susceptible to common diseases and parasites. Weaker fish will succumb to the high stress levels and die randomly.

High levels of H2S poisoning will be seen by a rapid, high number of deaths.

Hydrogen sulphide can be detected by the smell of rotten eggs emanating from the pond surface, the sludge itself or in the pond area. Anaerobic bacteria can convert nitrate back to nitrite and nitrogen gas.

I consider low levels of hydrogen sulphide to be a silent killer of Koi in our ponds. Many cases of diseases and ulcers can be traced back to poor filter maintenance and consequently continuous low levels of hydrogen sulphide. A constant lookout must be kept for possible areas where these conditions can gradually poison the pond and the Koi.

Chris Neaves


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