Sand filters have come a long way in the koi industry and believe me they do work very well. 

I can only shake my head and smile when I read how people complain about how dirty they get, mmm well that is why I like them, they work. 

One of the main problems I come across in the field where sand filters are used is that they have mostly been installed incorrectly. And they are not cleaned regularly enough. Each pond has its own regime.

Then sadly I also find that a lot of sand fil;ter installations are undersized for the the pond volume. A 3 bag sand filter with a .75 pump is only adequate for a pond volume of maximum 15000 Lt.