It sounds like whatever happened, it was sudden and because there has not been any physical damage to the fish, I would guess it could be a water quality issue.
It could be some chemical or other pollutant somehow washed into the pond. Has anyone sprayed fertilizer anywhere near the pond?
It could also be your fish waste that's gotten out of hand. This could happen if your filtration is inadequate or the pond's ecosystem is out of balance.
This is why without a good liquid reagent pond test kit, we can only guess. Get a test kit and post the results.
Do you have any plants? Plants are an important part of the ecosystem. They help filter out excess nutrients produced by fish waste. Lots of plants are needed. That, along with biofilm growing on everything and beneficial bacteria in your filter help keep things under control.
Describe your filter. Maybe it's inadequate.
You mention you run a media filter, fountain and aerator in the winter and hot days. This could be your problem. They should all be running 24/7/365 to oxygenate the pond and keep the beneficial bacteria healthy.
Could be a lack of oxygen that's killing the fish. Its imperative, especially when the weather is hot, to keep the pond oxygenated. Surface agitation is the best way to do that.
You mentioned vacuuming the pond. That's ok for removing the heavy sentiment on the bottom, but you want to keep all the growth on the rest of the liner in tact. That biofilm is part of your filtration.