We've had koi for about 8 years, and they're suddenly dieing. We
pumped out the pond, and put in fresh water. After waiting about a
week, we put in a new koi. It's been about 3 weeks. He appeared to be
fine, but we just found him dead. We made a mortar repair on the
waterfall in the spring, but we've done that before and never had any
problem. Also, we've done a complete water change since then. Any
ideas? We're puzzled and could use your help.