I have had my pond for 8 years and have never had any problems until a hail storm a few days ago. I went out and found them all dead. I then googled it and think it was due to the change of temperature and then it depleted their oxygen. Does anyone know or have a cover that would work for hail?
I have a pond with koi. We have had two hail storms (in ten years) that required replacement of our roof shingles twice which the insurance adjuster authorized; so serious hail. No problem with koi or the tropicals (mollies, danios, etc.)
I have inadvertently killed koi by covering most of the pond, slowing oxygen exchange. Hail hitting the pond would increase dissolved oxygen content by agitation and cooling the water. I strongly recommend using no cover. I also doubt it was temperature change, as my tropicals would be much more sensitive to cold than koi. The only thing I can think of is a very fast temperature change and that is hard to accept.
What about a lightning strike on the pond (grasping at straws)?