Last year the surface heater/deicer, and then the backup, on one of my koi ponds failed. Actually the second one developed a ground fault, but it's all the same since the circuit available there is on a GFCI. Under unusually heavy snow and very long periods of cold, this was the result:
Yes, there's a sixteen foot pond under there. All of my koi died, of course, probably from asphyxiation, four feet under the ice. No pictures of the carnage, as per usual.
This year, I did what everyone always told me to do, and dropped big airstones in. Three of them in one spot, because I'm paranoid. So far, so good:
