So results can vary widely. The inside of this bulkhead is connected to the submergible pump. When water changes are not being done, the valve is opened to provide a current flow in the pond. This provides exercise for the Koi, and actually provides a hole in the winter ice from the water motion. Black tubing is essential.
Clear tubing allows algae to grow on the inside of the tube.
This algae then filters the particulate material out of the water, reducing the effective clear tube diameter, eventually plugging the clear tubes completely. A quarter pound of food will feed 375 Koi fish inches. An 18 gallon plus a 55 gallon filter should handle 1 pound of food, or enough to feed 1,500 Koi fish inches. Two 55 gallon plastic drum filters should handle 1.5 pounds of food, or enough to feed 2,250 Koi fish inches. You may want a more sophisticated system.