Just this morning I noticed a reddish/purpleish mark right under my albino rainbow shark's eye. He's about 7 years old now and hasn't had any problems regarding his health and behavior in the past.
My aquarium is a bit over 50 gallons. It has guppies, swordtails, one ancistrus and one rainbow shark. He's territorial, but not to an extreme as some other breeds. Occasionally he'll chase away fish that get too close, but he lets the smaller ones roam around him. I have a feeling he might've bruised himself when or if he chased some fish off? From what I've seen he doesn't do it as much - and whatever happened must've taken place overnight because only this morning did I see the mark.
I'll attach an image of one side of his face and the other (hopefully I do it properly). Nothing like this has ever happened to my fish so I'm wondering if this is serious or not.