This is a good question and a hard one to answer. There is an optimum number of koi for each pond. The problem is, that number keeps changing. A well built 1500 gallon pond will easily sustain 30 koi 6 inches long. Two years later when those same 30 koi are now all 18 inches long you have a real problem, because your pond is now over stocked. Because people tend to overstock their ponds, all the Advantage Koi Pond System Kits, were designed to have an oversized filter on them. This means that the limiting factor for how many koi you can keep in your pond will not be limited by the filtering system. Rather, the limiting factor will be how crowded the koi look in the pond. Even with perfect water quality when the koi become too crowded their immune system begins to slow down and they begin to have problems. The best way to judge how many koi to keep in a pond with the Advantage System is simply to examine how crowded they look. If they can’t swim in a relaxed fashion it is time to remove some, even though the water quality tests fine.