Like were they a local morph, a colour morph, did any come in to the trade, why were some black etc etc..
Ah.
Gatf can turn black in dark conditions. I succeeded in turning a gatf black in a deeply tannic blackwater setup. It took months for the color to really show, but he lost it in a matter of days when returned to clear water and normal lighting. I'm not sure if it was the tannins or the darker condition of the tank because of the tannins that made him black, or maybe both.
I've heard anecdotal reports of "true" black gatf that keep their color in clear water being kept in Asia, but I've never seen anything that even vaguely hints that this may be true.
My personal hypothesis is that Douglas Dann's black gatfs- the ones we're all thinking of when we talk about this- were black because they spent decades in a river that he described to me as being extremely heavily tannin stained due to a large number of trees.
I expect if someone was willing to dedicate 20 years to a gatf in a tank with a peat filtration setup that's so intense that the water looks like sweet tea, that person would have a 2'+ gatf that looks just like the ones from that famous book.
I now live in Florida, so I can't be the one to do this experiment. Hopefully someone will. ?