I think you should absolutely do it.
I would use the red pipe diagram and extend the pipe further into the pond, close to the bottom, if you can. I have two reasons to make this suggestion. First, fish will not be as likely to swim out through the pipe if it sits two or three feet off the bottom of the pond. Plus, if fresh water enters the pond into the bottom, it will likely mix with the bottom water (depending on the time of the year) and/or possibly force some of the older water upward to mix with the healthiest water of the pond. Fresh water into the top runs across the top.
I would install an AgriDrain in the ditch.
