If we can identify the sources of organics and solids we can begin to control them and in this way bring some relief to the filtration system early on in the battle for clear water.

Solids in organic and inorganic form are the two main pollutants which cloud the water and prevent us from seeing our Koi. The cloudiness in Koi ponds is called turbidity.


Successful filtration starts with the control of organic and inorganic solids.


Pollutants seem to be a catch–all word used in Koi keeping for anything that is not wanted in the Koi pond!

This can range from fertilizer run-off from the garden, insecticide spray, dust, leaves, grass cuttings, etc.

The various pollutants found in Koi ponds are discussed within this document.

Inorganic Solids

Inorganic solids are introduced into the pond water from atmospheric dust, run-off from the garden or overhangs (roofs, bridges and pergolas), the “wash-off” from trees and the vegetation around the pond as well as by rain. If you have ducks on your Koi pond you have a problem with solids in your water. The ducks swim then walk in the garden and in their food and then swim again bringing large amounts of solids into the pond. Solids can build up and begin to cloud the water. The mechanical filtration part of your filtration system is designed to trap and remove (by flushing the chamber/chambers) solids from the pond.

Suspended Solids

These are solids that remain in suspension in moving water. The individual particles are generally invisible to the naked eye. Suspended solids consist of an inorganic fraction (silts, soil, dust, etc.) and an organic fraction (live and dead algae, zooplankton, bacteria, and detritus). Both inorganic and organic solids contribute to the turbidity or cloudiness of the water.

Decomposing organic material consumes large amounts of oxygen and contributes to the bio-fouling of the water.

Many suspended solids can be trapped in the filter systems but not all. When the fine suspended particles find a place to settle in pockets or areas of slow moving or stagnant water they become sediments or silts. This sediment can increase in depth over time and the deeper layers will be cut off from the oxygen in the passing water creating anaerobic areas that will produce toxic hydrogen sulphide.

Larger solids such as leaves will settle out rapidly on the pond floor or in the bottom drains or in the settlement tanks outside the pond. These organic materials must be removed regularly from the pond system as they are biodegrading, adding to the ammonia load and removing oxygen from the pond water.

Suspended solids can be very fine particles of minerals and dead algae cells.  Because of their very small size they are difficult to remove from the pond water. Algae cells will not all be trapped in the filters. Most will pass through back to the pond.

Bentonite clay can be added to the pond water to increase the flocculation of these fine particles making them easier to filter from the water. Bentonite is sold as Bentonite or montmorillonite clay. The Bentonite comes in two forms calcium Bentonite and sodium bentonite. The sodium Bentonite clumps more vigorously than the calcium Bentonite.

Although Bentonite is found in many clumping cat litters and Koi keepers are often tempted to use them in ponds, these are best avoided. Cat litters are in granular form. The granules clump (which is what they are supposed to do!) and can block bottom drains and piping. Using a powered Bentonite is far better in Koi ponds. Manufactures of clumping cat litters often add chemicals to introduce a fragrance to the cat litters. Some cat litters are sprayed with a plastic compound to reduce the dust associated with the clay. Other cat litters have colorants added for commercial appeal. As many of the cat litter additives are not declared, it is best and safest to avoid clay cat litters in Koi ponds.

Pots with soil and plants inside the Koi pond will be a target for the Koi. They will love the new roots or plant growth and they will dislodge the soil. This will become a major source of turbidity. There will be solids that will settle and fine suspended silts which are difficult to eliminate from the pond water and can be identified as a brownish colour to the water as opposed to the greenish colour of algae. Water changes are the best way to eliminate soil/silt from the pond. A good mechanical filter can also eliminate this pollution.

Suspended solids can also refer to small solid particles which remain in suspension as a colloid or possibly due to the motion of the water. Colloids are very fine particles that are dispersed within the pond water in a manner that prevents them from being filtered easily or settled rapidly.

Dissolved Solids

Dissolved Solids or Total Dissolved Solids (TDS) are a measure of the combined content of all inorganic and organic substances suspended in a body of water in a molecular, ionized or micro-granular (colloidal) form.

Many people define Total Dissolved Solids as solids that are small enough to survive filtration through a sieve the size of two micrometers. A micrometer, also called a micron, is one thousand times smaller than a millimetre. It is equal to 1/1,000,000th (or one millionth) of meter. Things on this scale usually can't be seen with the naked eye.

Most of the dissolved solids are a broad array of chemical contaminants such as calcium, phosphates, nitrates, sodium, potassium and chloride. Other harmful total dissolved solids can be pesticides.

Total Dissolved Solids (TDS) are differentiated from Total Suspended Solids in that the TSS cannot pass through a sieve of two micrometers and yet are indefinitely suspended in solution.


Organic pollutants take two major forms in Koi ponds – plant matter (in the form of algae, leaves, grass clippings, flowers, etc.) and fish faeces.

Organics in the form of algae and fish faeces can be the major cause of turbidity and pollution in our ponds that our filters have to contend with.

The organic load will vary from pond to pond, from region to region and from season to season.

The success of organic pollution control lies in the design AND maintenance of the system.

It is of the utmost importance to remove these organics from the system. The reason is that heterotrophic bacteria thrive on these organics and they grow faster and take up more surface area in the filter media than nitrifying bacteria.

Systems that do not adequately remove organics can and will crash resulting in the deaths of koi in the pond.

So the design of the pond is the beginning of solids removal.

Removing solids is significantly more difficult than bioconversion of ammonia and nitrite. Provision must be made in designing mechanical filters that are large enough to cope and easy enough to clean.

A pond that is clear and free of sediments would be a good way to determine if the organics and solids are under control.

Controlling Algae

It is critical to the biological balance of the pond and the resulting healthy koi that chemicals be kept away from the system as much as possible.

Biologically healthy ponds take time and are essential to the health of the fish.

Algae can be controlled using various chemicals or naturally using barley straw or barley straw extract. Caution should be used when using chemicals to control algae as some have high copper contents. These chemicals will also affect plants in the system and plants in your garden when back washing filters.

Suspended algae can also be controlled to some extent with UV lights.

Numerous studies have found that barley straw can control algae when used correctly. For the barley straw to work, aerobic conditions are necessary lots of oxygen). Therefore, how the barley straw is placed in the system is important. Do not over fill the bags so that the barley straw goes anaerobic and starts to smell.

After a time our ponds form a natural eco system. The string algae growing on the sides of the pond tend to predominate, the suspended single cell algae that turns water green recedes and the result is clear water.

There are seasonal changes in a pond and algae blooms and the water may turn cloudy in spring but clear a month or two later. So do not over react.

Over feeding can contribute to excess nutrients ending up in the water that algae can utilise for growth