Hi, I have had a koi pond for 20+ years and this spring I have lost 2 large koi in April and today, another is sick. The first one was at least 18+ years old and one of my favorites. There were no signs of injury and she appeared healthy. How long do koi usually live in a backyard pond? In a couple of weeks, another died and didn't have any injuries that I could tell. I checked the water and everything seems to be OK. I have plants in the pond and have been doing partial water changes as usual this spring. The other fish are eating and swimming normally. The age of the two other fish (the one that died and the sick one) are about 15+ years. Any suggestions as to what I need to do? Thanks