I've had a small pond set up in current location for about 9 months. I transferred my koi from another pond. Some of them are 10-12 years old, others are only 5-6 years old.
4 weeks ago, two of my koi (one older and one younger) died suddenly. Last week, another older one died. Yesterday, a younger one died and today one older and one younger will be dead within the next 2 hours or so. I have not sprayed anything on the lawn. I changed 50% of the water after the first two died. I use well water, so chlorine not the issue.
Weather was in low 80's °F (around 27 °C) when first 2 died. Now in mid 50's °F (around 12 °C).
I filter and have a small waterfall running constantly.
Anyone have any ideas why my fish are dying off?