You must first understand that you should have at least 500 Litres more filtration than actual pond water. This is because although the filter might say that it will work on a pond (lets say) 3000 Litres, if you pond was 2850 Litres then the 3000 Litres the filter would be struggling to work flat out to keep it clean.
You can never over filter a pond - so make sure the filter you choose is adequate. If you intend to have a highly stocked Koi pond then you should aim to have a filter to cope with one and a half times the capacity of the water in your pond.
To calculate the volume of your pond multiply the average length by the average width by the average depth and multiply the result by 6.23 for gallons. If you require this in litres multiply the result in gallons by 4.5
Once the volume has been established check the manufacturer's recommendations for how many gallons or litres their units will treat
Then decide which type of filter is suitable for your pond and its look at where it is positioned.
