Hello and welcome! So sorry! Thirty years is reaching the life expectancy of a koi. Maybe it was just his time to go.
Water tests “OK” really doesn’t give us enough information. Can you give us actual number results?
Your pond sounds like it’s just around or under 1000 US gallons. How many total fish are in this pond and have there been any fish added recently? That size pond is questionable for more than one koi.
It sounds like you are experiencing some serious water quality issues, and there might not be a one-size-fits-all solution. Fish can do fine until something tips the scale — such as a change in the number of fish or their sizes (the larger the fish grow, the more waste they produce), or a change in other conditions (weather....), so a pond that was fine a year ago can become toxic to the fish.
I wish I could help, but right now it sounds like you need to:
- test your water
- start water changes (use a dechlorinator if your water is treated)
- reduce the fish load