Beautiful pond! I love how when you put new koi in a pond, they are so skittish, so cute :)

Just how did you fill the pool with tapwater? Did you use a water breaker and spray it into the pool? Did you empty down the pool and refill with fresh tapwater?
What is the temperature difference between the pool and the tap water?
Normally if you just top off a pool with tap water using a spray through a water breaker
splashing down onto the pool you should have no problem. If partially emptying and then refilling you should consider using a water conditioner such as
Hikari's Ultimate

that you can add to a measured volume of water that is then added to the pool.

@Terrestrial man.

Short story version. I needed to add about 50gal of water. Turned on hose, should have left it on for about 10 minutes or so. Left for church, forgot to turn off water. 2.5 hours later, probably a 200% water change, untreated chlorinated water occupying 100% of pond. 14 dead fish.

Thanks for the tips. It was an idiotic error on my part that led to the demise of the fish.