Each pond is unique. Some ponds have more sunlight (read more algae) some ponds have greater dust and pollution settling on the pond. Some larger ponds may very well be clear and run for some time without flushing with a single filter. Calculations can be made regarding costs for different circumstances. For example – two 18 inch filters have 36 inches of surface area in their combined sand beds. The cost of two 18 inch sand filters is less than one 30 inch sand filter. We therefore, have more than double the surface area and two multi-port valves as well as more gravel which results in far greater overall flow through the system, longer intervals between back-washing and an enormous surface area for bacterial growth.

koi pond pump setup

Preparing a new Koi fish pond

Pumps and Sand Filters:

Generally speaking pump capacity is linked to the size of the filter by the manufacturers. This is not for the flow through the filter but to give adequate punch to the back-washing. Placing a ,55 kW pump on a 30 inch filter or even two 30 inch filters works very well when the water is moving through the gravel beds in the normal way. However, the back-washing is never entirely satisfactory because the smaller pump does not have the pushing capacity to completely lift and break up the gravel bed.

Therefore, the sand bed will clog more readily because certain sections compact over a period of time. (see the next section – tricks? for some answers).

For this reason manufacturers give the following pumps specification to sand filter dimensions:

18 inch = 0.6kW: 24 inch = 0.75kW: 30 inch = 1.1kW


There are some “tricks” that can be used to modify this ratio of pump volume to filter size. As discussed a smaller pump on a larger filter does not have the power to lift the weight of the gravel to break up the compacted top few centimeters. A 30 inch sand filter contains 200kg of sand for example. Opening up the filter chamber from time to time and manually turning the sand bed will break up the compacted areas of gravel that are clogging. Adding a Jacuzzi air blower at the same time as back washing will blow an enormous amount of air through the sand bed and loosen the gravel. Reducing the amount of gravel will not affect the mechanical filtration too much but will lighten the total weight and make it easier to flush completely.

In order to do this the sand filter chamber has to be opened from time to time. There are sand filter models on the market that are secured by bolts around the rim and there are models that have an easy access port on the side with a single butterfly release.

On the point of using a Jacuzzi air blower on a sand filter. The air blower can be added to the inlets from the pump and the gravel bed is given a good shake like a boiling kettle from the air. It is interesting to note that sand filters can be back-washed in the normal manner until the water coming out of them is absolutely clear. Adding the air blower immediately after this dislodges an amazing amount of solids that are still be trapped in some remote corners of the gravel bed.

Some pond keepers use zeolite in their sand filter with great success. Zeolite is lighter than gravel and will lift and backwash easier.


Points of Concern:

Sand filters are sometimes by-passed. If this is done for any longer than a day it is absolutely necessary to remember to back-wash the filter before it is used on the pond again. The water trapped in the chamber is cut off from the oxygen in the atmosphere. It will become anaerobic within a day or two. Large amounts of anaerobic bacteria are not healthy (actually they are a silent killer in our Koi ponds) for the Koi and should not be pumped to the pond. Anaerobic conditions can be detected by a very obnoxious smell.

A Bonus for Sand Filter Users?

It was mentioned earlier in this article that sand filters provide a very large surface area for bacterial growth. We know that as soon as the pond water moves there is bacterial growth on ALL the surfaces in the pond system, the walls, the floor, the piping, the waterfall. This applies to the gravel in a sand filter as well.

A cubic meter of 19 – 21 mm stone has 180 square meters of surface area for bacterial growth. A cubic meter of 10 mm stone has 360 square meters of surface area. A cubic meter of 3 – 5 mm gravel has 800 square meters of surface area.

A single 30 inch sand filter contains 5 bags of 3 – 5 mm gravel. This provides 130 – 150 square meters of surface area for bacterial growth. Therefore, two 30 inch sand filters will have considerably more surface area than a cubic meter of plastic rings for example.


High rate sand filters are very efficient mechanical filters. Efficient mechanical filters trap solids in a convenient place. The more solids that are removed from the passing water the quicker any mechanical filter will clog up. But the result will be clear water.

Koi pond filters have to deal with water conditions different from swimming pools. The pond eco-system is producing various organisms continually, the pond water is being polluted continually. In a swimming pool the living organism are killed with chemicals. Therefore, they are not growing continually.

Consideration must be given to using more than one sand filter on larger ponds. These must be in parallel so that system will benefit from improved flow rates because the restrictions from a single sand bed and a single multi-port valve and restrictions in the arms will be eliminated. This results in large surface areas of mechanical and biological filtration and extended time between back-washing.

The standard fine sand is extremely efficient at removing solids. Consideration can be given to replacing this with a slightly larger gravel such as 3 – 5 mm.

Using larger gravel than 3 – 5 mm will result in the arms at the bottom of the chamber blocking.

The gravel bed can be turned by hand occasionally. Easy access to the chamber will make this job more pleasant.

By-passing sand filters for any length of time allows anaerobic bacteria to grow in the chamber. The filter must be flushed before being used on the pond.

The sand or gravel in high rate sand filters has an enormous surface area for bacterial growth. The damage to the biomass by regular flushing is minimal as Koi keepers have run their ponds successfully for many years on nothing else but sand filters.