The only rocks I will put in my pond will be the expensive and sparkley kind when I try to hide them from my wife for telling me that I love my koi more than her!
The only filtration system I have seen that utilizes rocks is a system that belongs to a friend. He has a 10,000 pond that is stocked with at least 100 koi and the water is pristine clear. However, his rock filtration system is bigger than the pond itself. This lava rock filter then feeds into a bog filter that is about 30'x30' and is about 1-2feet deep. His whole yard is filled with ponds!!!!