define fantastic. Do you wish to filter the koi out of the water? Do you want to aerate the water at the same time? do you want physical, chemical, radiation, or biological filtering...or a combination? basically, pump water thru a screen. That should get you started. Koi will grow proportionally to the size of their home. If you keep them in a small area, expect small koi. If you grow them in a huge pond, they will...you get the idea. Plan your filter around the size of the water you are going to filter. I forget the rule of thumb for the ratio of water circulated vs total water..