I've been having problems the past couple years with my goldfish developing pop eye as the summer months come on. Most of them didn't survive but always a few survivors to breed in the spring and replenish the school. Last year I had 13 nice koi that were three years old and around 17". Then all of a sudden they started acting strange as the goldfish developed the pop eye again. Within two days I lost ten of my koi. Same pattern every year. All the fish are super healthy and active coming out of winter and then deteriorating into the summer. Every spring we get dozens and dozens (maybe even hundreds) of toads invading the pond each spring to spawn. I'm thinking with that many toads mucking up the water with sperm and eggs is making the water toxic. I've been a ponder for many years and all my water test levels have been perfect. I have a picket fence around the pond and I'm installing plastic netting so there are no spaces for them to crawl through. They'll have to climb 30" to get in, so I'm hoping it'll at least be a deterrent. Not going to buy any new koi this year while I see if this solves my problem. I've got two females and a male so I'm hopeful they will spawn me some babies. Should be an interesting year and hopefully I have solved the problem. Time will tell.