But the fish in the pic is not a koi. Agree since they all have it, it must be normal at least for them and if it was an issue, I think they'd be showing it by now.....
I went through my pics of light angelfish and I can't find something that matches your pics, but as I've said, I have seen redness at the pivot point when they are swimming a lot quickly. Like when a pair in the school is getting ready to spawn. Angelfish aren't 'built' for that type of swimming.
I also have seen redness like that under the skin in the face area as a result of fighting (in my case). It took maybe a few months to fade but eventually it did. It didn't seem to cause him any harm. I just increase water changes for a while like you plan to do
Keep an eye and let us know how it goes.
The leaves can influence pH if water hardness is already low, but 1 large leaf per 30 gallons won't change the pH much if at all. Oops I forgot you don't have a test. It would be good to know where your water sits at, but I guess better to be able to test the water before using leaves I guess as a precaution. Really water changes make the biggest difference. I like your idea to change 30% every other day for a while. It certainly won't hurt!