I was looking at some pond pumps at the koi centre last week and saw that the pumps are filtered multiple times. Made me wonder why. There was a 7 layered filter and pump which was HUGE.
I checked with the workers and they told me it was because the fish need a lot of oxygen in the water and a lot of clean water too apparently. The water wasn’t all that clear but I guess it was clean enough because the fish seemed happy 
How do you know your fish is happy anyway!