I'm not sure if I'm being a worry pot here, but one of my goldfish has developed a little black spot on his lip, and I'm not sure if this is normal. I've only been changing his water once a fortnight because each time the water's been really clean, but now I'm not sure if this has triggered it.
His health and appetite are normal, and there are no other markings on him (In fact he's in really good condition). I've included a picture of his marked lip.
p.s. he's in a 22 gallon tank.