poorly fish?

One of the new ryukin is very still and hasnt eaten yet. he seems to swim
fine, but stays in same place most of the time, hasnt eatien with the others
at all and I've just noticed a small dark spot on his tail and one on his
front fin. H'es a read and white and not sure if this coloration is just
colouring or if it is a disease?

Cant seem to find anything on it any recommendations, he doeasnt look to
active at all tho his gills are healthy bright red.

Tess