Yes, I got beckett Chlorine and Ammonia remover enough to basically treat the 1800 gallons. What behavior does a koi show with damaged gills? How much damage is deadly?
The damage varies so the behavior varies.
Little damage they could just act slow.
Little more damage and you can see gasping at the surface a lot, like staying at the surface, not just once in awhile.
More damage and they'd already be belly up.
Over the next few days extra mucus can cover the damaged area making respiration more difficult. So you could see health decline but I would not do any treatments. People will suggest all kinds of stuff, salt, slime coat stuff in a bottle. These can make things worst. I'm a little concerned about what was in the "water conditioner". Many of these increase slime coat by irritating skin. Not really something I would want to do to raw gills.But what's done is done and that's that.
Swimming around actively is a better sign than belly up. New water often excites fish toward spawning behavior, or they just plain like it. But active swimming can mean they don't like the water. That can induce jumping out of the pond. You can net the pond if you wanted. If these are expensive fish I would net the pond to be safe. I've never heard of like entire pond full of fish jumping out. Normally one or two, but always seems to be the fav or most expensive fish.
Another thing that's pretty safe is to add an air pump. That's just going to be good, damage or not. It maybe speed healing and increase immune system, maybe help over the next week.
I really would resist adding stuff (chemicals) to the pond. Especially if you test ammonia. That may have to be dealt with but freaking out and doing more water changes adding ammonia binders will make things worst. Better to assume the ammonia is already bound and see if it goes down over the next week which it should, but I don't know your water temp.
There's a kind of Catch-22 with this stuff, if you have to ask people what to do you really shouldn't be doing those things. Fish are very tough. It's cool to learn about this stuff when these events happen but more for the next time. Hearing something and then doing that thing and then learning what it is you did is not good. Very common, but not good.
Because the fish aren't belly up, you're not seeing any clearly distressed fish yet, my money is on the fish being fine. The can take some damage and still appear normal, just like us.