Thanks for the comments, everyone. He's on a diet of various Hikari pellets, HBH spirulina pellets, and frozen bloodworms and brine shrimp. I think the key to his nice fins is that he's been housed alone for a while. I plan to get a school of rosy barbs soon but he needs to settle in first. He loves having barbs to keep him busy.
I have bred him before, to a very pretty female marble. He wasn't very nice to her so I had to separate them, but they had one batch of fry together. I admit, most of the fry were very plain with only a couple having a significant amount of markings on them. None of them were as striking as him, and I picked him out originally because of his markings.
Jimmy side eye, your male marble is gorgeous! I wish my male could grow a nuchal hump like that, but his has always been very small even when he was housed with the female. Mine's temperament isn't aggressive at all, except to female cons and plecos. He's curious about dithers but has never attacked them, and whenever I used to house him with other cichlids he always got picked on. I think he does better alone.