fish do not help each other. they have no feelings or emotions towards other koi. Not sure what they are doing, other than doing what instincts tell them to do and that's to stay as a school because there is safety in numbers.

I disagree. Fish may not be human or mammal, and their emotions may be primitive not nuanced like ours. But they certainly do have recognition of and connection with other individual fish - some seem to be friends and hang out with each other. Last summer I put a fish back into the pond after he'd been sick and you cant tell me he didn't feel some fishy equivalent of joy as he went racing around greeting his pals and checking out all the familiar nooks and crannies of the pond before settling down and getting back into grazing on the algae with the rest of them. It was fun to watch.

That said, the survival instinct is at work too- I have seen a similar thing happening where they were nudging a sick sluggish fish to keep her moving I think for reason above poster mentions - safety in numbers, and a sluggish fish would attract predators.

Aside from the water issues, they could be mating. It's like the running of the bulls when they are trying to get the female to release eggs. How do you know your water was 'toxic'? Did you test it?