Just a quick update...
It hasn't been all flowers and sunshine. I was out of town last week and found a baby red head geo I had inherited (ironically enough, from a friend who's discus were being bullied) developed a taste for stingray. He chewed up one of my marbles a little bit. He's since been removed (and fed to my monitor). It's been a few days and the ray is healing up just fine.
My initial fear wasn't aggression, but the dreaded marble-rot. I've been keeping a close eye on the tank and can safely say that's not the case, thank god. I'm hoping he's 100% healed in the next week or two.
These pics are from earlier in the week and the wounds have already healed up incredibly well.