Question
I have had a pond (1000 gal)for about 7 years. Most of the time I have had very healthy goldfish and koi. About three times in the seven years we have a had a big hail storm--this pretty much wipes out my koi and sometimes my gold fish. These fish have lived thru blizzards--then a hail storm and they die. I test the water and it is always good for ph, nitrites, ammonia and salt level. I have not tested right after the hail because of time constraints or i have been out of town though I'm prepared to do it if it happens again. Any experience with this problem?

Answer
Wow, I've not heard of total die offs from hail specifically.  I think that with hail storms, the barometric pressure is particularly low and that the storm causes a reduction in the oxygen levels in the pond.  Hail ice and rain have little oxygen in them.  They also don't have any ions or minerals to buffer the water.  A lot of rain or hail results in both low oxygen, possible pH, low alkalinity and hardness, and sometimes a pH swing (fast change, normally down).  If you could get test kits for pH, hardness, alkalinity, and oxygen, you might find something gets thrown off right after the hail storm.  To reduce problems, test those water quality factors as they are normally and adjust them if needed.  Aerate the pond well, especially during storms.  Of course, right when the oxygen is lowest after a heavy rain is often when the power goes out which makes things much worse.  Physically, hail can rip holes in plants and might rarely bonk a fish on the head but otherwise it's effect should be similar to that of a heavy downpour.

Good luck!