You will need to net the one floating on its side and see if it has injuries, or any obvious problems. Outside of a parasite, I'm thinking since all your water tests were ok, a predator may have entered your pond. Your other fish may not be sick, but frightened?
Just a guess. Good luck.

What about the heat and the amount of dissolved oxygen in the pond? That was my problem last summer. There was nothing else apparent that could be causing problems. But it was hot hot hot at the time. My big koi was the first to show any signs of stress. After moving him in the house where it was much cooler and setting him up there, he recovered. Since, I have been VERY mindful of the heat, the little pond has a sun shade up over it plus floating plants now.

Thank you for the advice. Our pond is shady and cool. When we go to net the fish that is laying over, it swims off like nothing is wrong. I see no damage or sores?? It is so confusing to know what to do. We will be salting again tonight and then see what happens. The fish seem to be perking up a little with the last salt addition. Keep sending the advice. I really appreciate it!
Pond lover.

Ok, I believe the formula is 1 teaspoon salt per gallon of water 3 consecutive days. I wouldn't recommend any more water changes during this period. The formula is a gradual increase so it doesn't burn your fish.

Re-check your water. And don't try to feed them if they'r not eating. A few days without eating won't hurt them.

It seems maybe the salt is helping. That's good news. :)

THE SAME THING IS HAPPENING TO MY FISH. I ADDED SALT AND DID EVERYTHING YOU DID.TOMORROW I WILL PUT SHADE AROUND MY POND AND SEE IF THAT WORKS.I THOUGHT I WAS THE ONLY ONE WITH THIS PROBLEM.ITS BEEN A YEAR SINCE YOU WROTE THIS. I HOPE YOUR FISH GOT BETTER AND SO WILL MINE. THANK YOU. ANYTHING ELSE YOU KNOW THAT CAN HELP MY PROBLEM PLEASE LET ME KNOW.