blue eye plecos with spot??

[ohmar606]

I got these a few months ago, I believe in the spring, I didnt notice this until recently one of my plecos, has a spot on his fin, is this enviromental or injury related , my other 2 did not develop this spot.
Any ideas??

[wingfire1]

I read somewhere that this is genetic. If I can find the page agin I'll put the link up for you.