Welcome to our group, but sorry that your introduction started out with fish losses.
You might want to do more frequent water changes -- most do them once a week, or at least every 2 weeks. You mentioned that you go by when the water is clear -- but water clarity is not an indicator of water quality, though that is a common misconception.
And it's not easy keeping everything healthy when your pond is as small as yours is, so you've been doing something right all these years! It is discouraging, but don't give up!
Oh, and BTW, when you feed the pellet food, you can crunch some of it up into smaller particles if you think they are too big for your gold fish to eat. I don't have koi, but that's what I do when I have smaller gold fish and I want to be sure they get their fair share.