The purpose of the pump is to supply water to the filter for cleaning. So, the two must be well-matched. To choose the right size pump, you will have to calculate how much water you have to move. Depending on the size of your pool, you should aim to move the water through the filter every two hours. If your pool is heavily stocked, increase the rate of flow through to once every hour. Koi produce about four times the waste as typical pond fish. Therefore, the size of your koi population is an essential factor in deciding what size filter and pump you need.