The pump has been bought. It can deliver 260 litres/ minute. I decided to use a submersible "dirty water" pump so it will be able to handle any muck that happens to be at the bottom. I am assuming the flow will half, partly due to the pipe diameter is going to be reduced from its current 2" and also partly due to the effort the pump has to push the water through the biological filter. So let us say the flow is about 150 l/min therefore it can turn over roughly 9000 litres per hour . That's at least the volume of the pond once an hour. This is the minimum recommended bench mark according to Koi breeders having a pond this size or bigger. I decided to try the pump in a foot of water and its output was impressive. In a way lucky I did because the next day I realised the water had an oil film. I should have thought of it, obviously the pump is lubricated before shipping to stop corrosion but it would have been a little disaster if that went into the pond. I must cycle it a few times prior to installing it...
