Question
We have had our six koi for almost ten years.  Bought when they were about three or four inches long.  Now, the longest is long, thin and sleek - about eighteen inches long.  Our largest and heaviest koi was only about a foot long, but was one hefty fish!  About five pounds.  I know because I had to take it out of the skimmer.

Four nights (on Wednesday night) ago we had two five minute hail storms about 45 minutes apart - 2am and 2:45am.  Each separate storm lasted about five or six minutes.  The next morning the pond was filthy.  I drained as much water as I could from it and refilled the water, adding Stress Coat.  All the fish were obviously stressed.  

Friday I could only see five of our fish, so I checked the skimmer.  Our largest koi, nicknamed the CowFish because of color and bulk, was dead with its head out of the water.  Our pond specialist came and checked for pollutants, etc.  All fine.  He said he thought the others were doing okay, considering what had happened.

Yesterday another fish was obviously more unwell than the others.  Today, I think it too is almost certainly dead.  It hasn't moved in over six hours, as least that I can see.  It is "parked" behind a rock and a plant.  I don't think the current from the falls and the pump can move it.  I've called the pond specialist again because I cannot physically get to and remove it due to hip and rotator cuff issues.  And I know it cannot just lay there and deteriorate because of the effect on the other fish.

A third fish is now among the very ill.  It is a gorgeous red on the bottom and black on the top, about eighteen inches long.  As I watched several times today, as it was swimming it rolled over in the water at least twice on each trip.  Might it make it??

None of the four that are still able to swim (discounting the wedged one) spend much time out in the open water.  Instead they stay huddled under the shelter-rock we had installed to give them protection from great blue herons.

I understand about the impact of the drop in temperature, lack of oxygen and the effects of debris.  

Is there anything else I should have done?

Is there anything else I can do to help the remaining fish?

Is there a possibility that all will die?

How long might it take to have assurance that whatever fish that are left will survive the ordeal?  A week?  A month??

Thank you for any help/advice.
Maryann

Answer
Hello Maryann!
So sorry to hear about your Koi!
Hail can kill Koi for a variety of reasons: rapidly changing water temperatures and/or pH, ingesting hail pieces, contaminants in the hail (this could be things your pond specialists cannot test for) or they could have even come close to the surface thinking it is food and were struck by the hail.
I would do several partial water changes (1/3 at a time) adding stress coat, beneficial bacteria and pond salt (do not use table salt). Make sure they have plenty of oxygen! There is not much else you can do, unfortunately. You should know after about 2 weeks if the rest of the Koi will be okay.