Thank you very muchBut 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!
Thank you for the infoOn other websites, I have seen how angelfish breeders advise for this. Supposedly the red lines and markings are due to "spills", they can be due to several factors, the first is the PH, if you have an incorrect PH it can trigger a bacterial disease in your angelfishes called septicemia. This disease can also arise after a poor quality in the water, check your levels of ammonia, nitrites and nitrates, then it becomes factor 2. The third factor is a sudden drop in temperature, as you can read, they are refer to the general quality of the water, check these points, discover the fault and eliminate it.
Thank you for the infoI've seen similar red areas in koi angels' pectoral fins, but not on the face... They didn't seem to mind...but I'm not sure they lived as long as they might. They were gift fish & I was unsure of their age. They did breed a few times over 3 years or so, for what that's worth.