Koi Dying - Please help
I live in Green Bay, Wisconsin and have a pond (approximately 15,000 gallons) with 21 Koi. The pond froze over during the winter but we did have an aerator running. When the pond thawed the fish looked healthy, upright and moving around. A few days ago one of the fish started dying. There was no apparent reason as the color, fins, eyes, etc. looked good. Today, another fish died, again with no apparent distress. As of right now, I'm removing some of the water from the pond and intend to add more from the well. Can anyone provide any assistance in diagnosing and resolving whatever might be causing this issue? Thank you so much! I'd hate to lose any more of them!