The typical way to filter your pond mechanically is to use a sand filter. Quite a lot of pond keepers will tell you that a sand filter is an absolute no no and should only be used in swimming pools combined with chemicals. We believe that this attitude derives from many web sites written by dealers who are trying to sell their high tech bio filters and therefore break down the use of sand filters.

Disadvantages:

If filter sand is used, you will definitely have numerous problems with clogging of the filter. Use bigger gravel to solve the problem.

If you bypass your filter for longer than a day (due to lack of oxygen) or do not backwash regularly, you might have problems with nasty bacteria. It is crucial to backwash your filter if it was not running for a few hours.

There is a fairly big initial capital outlay because you need a stronger pump ( R 1100.00) (to backwash the filter effectively and push the water through the gravel bed) and your filter will cost you approximately R 1000.00 and your gravel will cost you approximately R 300.00 ( R 100 per bag) A submersible pump will not work.

Advantages:

It is an extremely convenient way to keep your pond crystal clear by a simple backwash once a week. You can do this even on your way to work with your suit on and takes only a few minutes. At the end of the day, you want to enjoy your pond and do not want to clean huge dirty filters by hand, which is sometimes a mission.

The average sand filter provides approximately 150 square metres of surface area for bacterial growth which in itself is a form of a bio filter and is normally sufficient for a 10 000 l pond. Combined with a small bio filter filled with for example alfagrog or lava rock, you will have sufficient mechanical filtration and enough bio filtration for even a 20 000 litre pond provided that your stock level is not too high.

It can be combined with a settlement chamber where all the major dirt settles in the chamber to be flushed out from time to time, before it even reaches the sand filter, leaving maintenance to the sand filter to the minimum.

Damage to the biomass(beneficial bacteria to be maintained) is minimal with backwash.

It looks neat and can be buried or placed far away from the pond.

Compared to the latest bio filters, the capital layout is not that big and it is very durable, lasting for many years.

It is easy to bypass in case you have to medicate, using the simple valve. Before you resume, do a proper backwash to avoid problems with nasties.

Although it clogs up from time to time, it can be opened easily to enable you to stir any crust developed at the surface before you backwash. For this purpose, the Koi filters have a lever to easily open and close it, as opposed to the swimming pool filters that have to be unscrewed.

It comes with a valve to easily channel the waste to your drain by using the waste feature, especially if you attach a vacuum to the inlet.

The main function of a biological filter is to create bacteria which break down the hazardous ammonia in the water deriving from fish faeces into Nitrites which in turn breaks down to Nitrates. The name is thus misleading because it does not really act as a filter, but traditionally the biological filter had a dual function as it also acted to filter debris. However, in our modern society, most people use mechanical filters like sand filters or settlement chambers combined with slow flowing chambers with media to clean the water from debris before it reach the bio filter. The more fish in the pond, the more ammonia and nitrites are being created and the bigger the bio filter required.

The bio filter is the most important filter of all and is often neglected. If it does not work properly, there will be no bacteria to break down the ammonia and nitrites and the fish will die.

A bio filter is any system that creates the right type of bacterium (Nitrosomonas and Nitrobacter) required to break down ammonia and Nitrites. It normally contains a container with bio media inside, and the water flows over the media into the pond. Waterfalls also act as bio filters but are normally not sufficient.

Bio media is the media placed inside a bio filter to create an ideal environment for the bacteria to grow. The best on the market is Alfagrog which contains a lot of air pocket which is ideal for them to grow in, or lava rocks, which does the same. Plastic sponges or plastic bio balls or even plastic hair curlers are quite popular, but they need at least twenty times more media than lava or Alfagrog.

The bacterium required needs a surface to cling on and with ammonia and oxygen they thrive. However, it takes at least one month for them to grow, and therefore, a biological filter is not effective within the first month (depending on circumstances), and it takes at least three months for it to function properly. Only after about 12 months, it is fully mature and runs at full strength. Therefore, it is always advisable to refrain from placing to many fish in a pond with an immature bio filter. Rather start of with a few fish and gradually add more fish in accordance with the growth of the bacteria and the maturity of your pond.

It is possible to boost the growth of bacteria by placing some mature bio media from a different pond into your new filter. This will stimulate growth and mature your filter in a much shorter period. There is also products for sale to "seed" your filter. Ask your local pet shop.

The bacteria also grows inside your mechanical filter like for instance a sand filter, against anything it can cling to in the pond, and even on the liner of the pond. However, this is normally not enough, except if you only have a few fish.

Because the bio filter gets dirty from time to time due to debris escaping your pre filter, it should be cleaned to prevent the growth of unwanted bacteria. This should be done by ONLY using pond water because any Chlorine in normal tap water will instantly kill the sought after bacteria, and rather stay away from Chemicals as far as possible. Cleaning should be done every three months depending on your system and the effectiveness of the pre filtering system of your pond.

Size: The larger, the less often you have to clean it, and remember, you can have it too small, but never too big, and leave room for extensions.

One of the easiest ways to create a very effective filter is to add a concrete filter to the main pond, connected to the bottom drain. The first chamber, being the settlement chamber, is where most of the waste gets trapped because this chamber is directly connected to the bottom drain. Then the water flows over to the bio filter chamber from where you pump your water and pulls water from the settlement chamber. From here, the water goes to your sand filter (after most of the waste have been trapped) and then to your UV light and back to the pond. In the first chamber you add shade cloth or any media that will trap the waste. In the second one (or third and fourth), you add bio media like bio balls, spawning brushes, Alfagrog etc.  The more chambers you add, the better the filtration and the more bacterium you will create to get rid of Ammonia and Nitrites. You have to have a waste outlet connected to your main drain on both chambers to enable you to flush the waste from time to time. Obviously, this filter must be on the same level as the main pond, because it works with gravitation.

Photos of one of our two chamber systems that works very well:

(The second chamber can be split into two with a fiberglass sheet or other non toxic sheet to improve the effeciency thereof, in effect creating another chamber)

The flow rate of the water through this filter should be low, so that the waste will be trapped successfully. If the flow rate is too high, the waste will go through  the media in the settlement chamber, through the media in the bio filter chamber and back to the pond. Therefore you should divide your suction between the filter and your skimmer box to lower the flow through this filter. To do this effectively, you have to have a valve on the pipe connected to the last chamber. (See second photo)

Diagram of a three chamber system:

One can also have only two chambers, but the more, the better. The chambers don't necessarily have to be directly attached to your main pond. It can be added afterwards, as long as you can link your bottom drain to your first chamber, and the surface level must be the same as your main pond.

See an example of one of our chambers away from the pond with this fiberglass mould and separate concrete chamber.

Click to enlarge

Another cost effective option is to do your chambers with plastic containers as follows:

The chambers can even be done with normal 90 liter waste bins if your water volume is below 4000 liters.

It is important that each chamber can be flushed to get rid of the waste on a regular basis. The best is to link the bottom of each chamber to a waste pipe connected to your main drain, with a valve that can be opened anytime. Just stir the chambers a bit and open the valve until the chambers are clean, alternatively it can be connected to your pump with a valve, in which case it can be flushed with the pump, by putting the multi port valve on your sand filter on "waste".

The advantage of this system is that most of the waste gets trapped in the chambers before it reach the pump and sand filter. The sand filter is only there  to remove the very small particles that has escaped through the chambers and  to "polish" the water before it returns to your main pond. In this way, it limits the maintenance on your sand filter and avoids the dangers of bad bacteria growing in your sand filter due to lack of oxygen in the sand filter caused by too many sludge.

You should limit the flow of the water through the UV light to approximately 4000 liters per hour(30 watts unit). If your pump provides more than that, which is usually the case, you should bypass the excess water directly to the pond by installing a splitter.

Budget: High pressure pump (R 1100.00) + 2 bag Sand filter (R 1100.00) + gravel for sand filter ( R200.00) + bio media ( R 300.00) + plastic containers/chambers ( R 600.00) + connections (R 600.00) + piping (R 200.00) = R 4000.00 (UV, skimmer, bottom drain and valves excluded)

The sand filter can also be left out and added at a later stage depending on your budget.  If you do away with the sand filter, you may also replace the high pressure pump (required for a sand filter) for a much cheaper submersible pump in your last chamber that only uses 150 Watts of power as apposed to 400 Watts used by a high pressure pump. A submersible pump that delivers up to 8000 liters, should cost you no more than R 500.00.

Small pond up to 5000 liters:

If you have a small pond and a low budget it is not advisable to use a sand filter and swimming pool or other high pressure pump because the high flow of the water is going to create too much turbulence for the fish and it is not worth the extra expenses.

Do away with the high pressure pool pump and sand filter, and replace it with a submersible pump in the last chamber and build the filter chambers with three waste bins (or 2 or one that you split up in different chambers) or similar containers connected to each other as indicated in the diagram above. In this way, your expenses will be a fraction of the sand filter route. A good submersible pump with a flow of 8000 liters per hour will only cost you R 500.00 as apposed to the R3000.00 for the sand filter and high pressure pump required for a sand filter.

A lot of people believe that if they have a sand filter, they do not need a bio filter. This is not true. A sand filter does act as bio filter also, but it is just not sufficient. A separate bio filter filled with bio media is an absolute requirement and much more important that a sand filter.

If it is impossible to have a bio filter chamber/s on the same level as the pond, you can always pump the water to the bio filter chambers that is above the pond level. From there, the water can flow back to the pond with gravitation via your water feature or waterfall. 

Each chamber should have a drain hole with valve at the bottom to enable you to drain the debris and sludge from time to time.

The plastic containers can be connected directly to each other with a tank connector to save money. They are available from most hardware stores and should be at least 50 mm in size.

DIY Plastic chambers filled with bio balls, lava rock, hair curlers, shade cloth, kaldness filter bushes, alfagrog or a combination thereof, acting as your bio filter:

If the water flow from the submersible pump is too high to flow through the chambers, and it probably will, it should be split to enable some of the water to bypass the chambers and can be connected to a venturi or aim flow in the pond. Approximately 50 % of the water then goes through the chambers and 50% to the venturi or aim flow or other water feature. If the flow through the chambers are too high, debris will flow right through your chamber filtration system and back into the pond, and it will deceive the object.

Budget: Submersible pump( R 500.00) + three plastic containers( R 300.00) plus connections (R 200.00) + piping ( R 100.00) + bio media ( Total: R 200.00) = R 1300.00 (UV Excluded, skimmer, bottom drain and valves excluded)

UV Light

Unless you are going to have only a few Koi and a lot of plants, a UV light is essential in South African conditions;

In very simple terms, a UV light is a waterproof Ultra Violet light, normally in the form of a fluorescent tube. The water from the pump flows over the tube towards the outlet of the pond and gradually kills most of the algae in the water.

A lot of Koi keepers believe that the UV light also kills bacteria. This is not true as the water flow is to fast to have any effect on bacteria flowing threw the unit.