Solid Wastes A Major Source of Pollution

  The Fish Environment

Dealing with pollution

In an active koi pond we have two types of pollution; dissolved and solid. If we could remove the solid wastes from the system before they had chance to dissolve and pollute the water we would have better water quality, less dissolved pollutants and fewer health problems.

Solid wastes and koi health

If we summarize the situation so far, we can see that if we are to maintain the status quo as far as water quality is concerned, we need to remove the pollutants at approximately the same rate as they are produced. We have also seen that the pollution is basically in three forms: dissolved compounds, such as ammonia, inorganic pollutants such as phosphate and DOC, and solid particulate waste.

Unseen but there!

Solid waste will ultimately be broken down by decomposer microorganisms into a wide range of dissolved pollutants, adding to those already in the system. It makes no difference where in the system these solids decompose – the end result will always be the same, that is, further pollution. This is an important point as many koi-keepers think that once solid waste is out of sight (in the filter) it is no longer a problem.

With the rapid throughput of most filters, the dissolved pollutants produced as these solids break down are quickly pumped back into the pond. What we really need is two filtration systems – one that enables us to remove the solid wastes from the system before it has time to pollute the water and the other to deal with the dissolved pollutants. After all, if we could remove waste solids from the system, we would prevent most of the sources of pollution.

It doesn’t matter whether the solid wastes decompose in the pond or the filter – the result is the same – polluted water!

So perhaps, we should look on our filter as a system of two parts, one part dedicated to removing solid waste matter from the system (not just the pond) and the other removing dissolved pollutants.

Remember, too, that the pond is also part of the filtration system, as a significant amount of mineralisation and nitrification will take place on surfaces within the pond. The pond will also act as a settlement chamber for solid wastes, which will need regular removal to prevent them polluting the water. It is my opinion that the regular removal of accumulating solid wastes presents the koi-keeper with his or her biggest challenge and a great many problems will be avoided if this can be done effectively. For regular disposal of solid waste there are essentially two practical options: settlement and entrapment.

Settlement areas or chambers have to be fed by gravity-flow systems, ideally via a bottom drain. This way, solids are moved gently to a collection area, ready to be flushed out of the system. The traditional method required a large settlement chamber and these can be very effective provided that the chamber is large enough and the flow rate is low enough to give lighter solids time to settle. The retention time for water in the chamber is important. The retention time is simply the filter volume divided by the flow rate, thus:

Retention time =filter volume/flow rate

For instance, if a filter has a flow rate of 2,000 gallons per hour and the settlement chamber holds 200 gallons, then the retention time will be:

200 gal / 2000 gal per hour = 0.1 hour = 6 minutes.

It has to be said that, in the above example, the short retention time of 6 minutes is unlikely to be satisfactory, whereas a chamber volume or capacity of 300 gallons would give a retention time of 9 minutes, allowing much better settlement.

Having collected solid wastes, it is important that they are flushed out of the system regularly, before they have time to decompose. During summer this could be as often as twice a day, and obviously less frequently in winter. This means that the settlement chamber will need to have a drain to facilitate easy flushing to waste.

To maintain good water quality It is essential that solids are removed from the pond and filter before they have time to pollute the water 

A better way of collecting solids

An increasingly popular settlement option nowadays is the cylindrical chamber with conical base that promotes a slow swirl of water. The cylindro-conical shape encourages settlement of wastes into the bottom of the cone, where they collect together, making removal to waste simple and efficient. Again, retention time is important and a slow throughput will be more successful than a chamber that resembles a vigorous whirlpool. One should be guided by the manufacturer as to the right size for your systems but, if in doubt, err on the large side.

When set up correctly these chambers work well and are probably superior in practice to rectangular settlement chambers of similar dimensions. Both types need additional cleaning if solids are not readily flushed to waste since solids may cling to the sides and there may be areas of poor water flow or ‘dead spots’.

Last but not least is the pond itself. Even the best-designed pond seems to have dead spots, where mulm and fish waste collect. Any waste that isn’t drawn through the bottom drain will need to be removed from the pond before it pollutes the water and there are several options, depending on the pond design. It could be gently pushed towards the drain with a soft broom; the waste could be carefully removed with a fine net; or the pond could be vacuumed. In most cases it is probably a question of combining all three actions, with most ponds benefiting from a regular vacuum during summer.


The most common entrapment systems consist of filter brushes or sheets of foam. We should consider what type of bacteria likely to be attached to the brushes or foam, which are heavily loaded with trapped solids? Common sense tells us that it is going to be heterotrophic bacteria, and do we really want to encourage high levels of these bacteria in any part of the system? You will recall that many heterotrophs are also opportunistic pathogens and are quite happy to lunch on our koi – given half a chance!  Obviously the answer is No. So the important thing with entrapment is that the entrapment media are kept clean, otherwise they themselves become a source of pond pollution.

Any trapped solids must be removed from the system on a regular basis, otherwise they will simply decompose and pollute the pond. They will also encourage high levels of opportunistic bacteria.


This reminds me of a case last year, when several fish in a quarantine tank became ill. The tank was spotless yet the fish had parasites and were suffering from the onset of bacterial problems. Further investigation showed that while the tank was exceptionally clean, the filter wasn’t. When we took the media out for cleaning, the smell was overpowering. The media were covered in a yellow slime which, of course, was all solid fish waste slowly rotting down. In this case, the filters were slowly poisoning the fish. Following a good clean-out of the filters, the fish were soon back to normal

By using a combination of settlement and entrapment it is possible to remove a lot of solids from your pond before they rot down provided, of course, that these areas are cleaned regularly. If we are successful in removing solids from the pond before they pollute the water we are part way to achieving the ideal of unpolluted water.

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