Angelfish. Red sore like spot

[Sleeveless]

Hey guys. As the picture shows. One of my Angels has a red patch on its side. I’m thinking injury but not sure. Sometimes the other angel nips at him but this one does it back. Other Angelfish doesn’t have any marks or issues. You can see it on the low right rear in the pic just above his fin

 

[Sleeveless]

Water is

PH 7.8
Ammonia 0
Nitrite 0
Nitrate about 20ppm