Black lines/streak on goldfish going on 3 weeks

[moogie21]

1 of my goldfish has developed this black streak/lines for the past 3 weeks or so. and other 2 looks like they have fin rot. i checked the ammonia/nitrite/nitrate and all appears to be 0.

a week ago, i started to put all 3 in quarantine tank. i raised the temp from 74 to 78F. added aquarium salt, melafix/pimafix. a few days ago i added kanaplex. still no improvements.

he seems to act normal the past few days but today has been laying in the corner

any other suggestions? what could it be?

thanks

 

[ember04]

Could be colouration of the fish it’s hard to tell from photos but my gold fish change colour as they age