red growth base of fin

My male 2 yr angel has red growth at base of one pec fin. Acting normal, ie swimming, eating, chasing girlfriends (3 other angels in tank, all females). Water parameters all good (7.5/0/0/30 pH/amm/nitrite/nitrate). 75 gal. Tank is 2 years old. Noticed it 2 days ago. Only thing different is...