Update.
The little guy has grown well, currently at about 10". Loves its Ziegler pellets. It's gotten quite thick too.
Despite all that, I've been having a problem with it. As you might have noted, there is a small fuzzy white thing on its anal fin in the photo above. I've been having difficulty with it. It looks like kind of a fungus and it responds to Malachite Green treatments rather well. Yet soon after it makes a comeback.
The dat is in a system of four 240 gal tanks with hundreds of fish sharing the same water and none others have this problem.
Having done I think 3 or 4 Malachite Green treatments, I stopped doing them about 5-6 months ago to see what would happen long term. The fuzzy clumps came back slowly. They vary in number and size but are always present on the fish, which, notwithstanding, has been behaving like nothing's wrong.
Can anyone recommend anything? I cannot separate the fish right now due to not having a hospital tank at the moment plus chilly weather outside. I can give it baths, which is of course stressful for it. Or treat the entire system, but then I will be risking other fish well-being, just like I have done with the pretty harmful Malachite Green before, which miraculously didn't seem to affect other fish. I was very careful with it.