Foolish question but is oxygen crash from rapid temp change or inflow of water a possiblity and did you have any amount of time where pond was covered with snow and ice?
We haven’t had much moisture this winter, or spring. No big inflows of water. Temps are a bit on the crazy side with dramatic ups and downs. So maybe that happened.
It was frozen over a few times, but no serious snow, and it warmed up again pretty quickly.
