our MG Vlad has thin light lines on the edges of his gill covers. at first i was worried it was a fungal infection, but on closer examination i found that he's been coloring out a lot since we brought him home (he was originally mostly yellow, but in the last week his body has turned mostly blue) and the lines were just part of him that stayed yellow. it might be a natural color change, they happen a lot, particularly with younger fish (you said you've only had him a few months, so he might still be growing into his colors)
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)