Bear in mind that filter size and water exchange rates are highly variable for each situation. If your pond receives a lot of tree debris, or you have many fish and feed them regularly, you also need a massive filter to deal with the organics. Preferably an easy to service filter to occasionally remove what the filter has accumulated. Otherwise the filter reaches a saturation point and things begin to fall apart. I don’t know how to help with sizing a filter for outdoor use. Aquariums, yes.

I’ve been following a natural swim pond group, and am amazed at those systems. People generally don’t put in fish to avoid the load on the large natural filters that are not easily serviced to keep them usable longer. However, at some point, nature will win, and the filters will need to be cleaned out at great expense as they are just very large reservoirs of sand and gravel with some bog plants to take up dissolved nutrients.

But I am soooo tempted to build one of those to expand my garden/lily pond near the house.