Here is the latest diagram of my pond with new pipe layouts and UV added. Also updated valve locations and dimensions. The 3 way valve near the skimmer is usually open to both the skimmer and bottom drain in normal operations. I can close the skimmer while feeding. The 3 way valve after the pump is usually closed to waste during normal operation but is closed to the pump when cleaning the bio filter. If a large water change is needed I can pump to waste. The UV Sterilizer valves are usually open while the main pipe is closed. The UV circuit can be shut off so I can remove for maintenance or storage. The veggie filter also has a drain line not pictured.
NOTE: This is not an ideal KOI pond. This is what I built with my budget constraints. If looking for how-to do it right try koikichi.com.