the largest "black" shows up at 6cm/2.36" Black widow tetra Gymnocorymbus ternetzi, now as for active...mine seem happy to lazy around and only go into hyperdrive when its feeding time. 6 in a 20 long term may not work, and it really boils down to each group of fish..mine would be fine there due to them being lazy fish.
Black skirt tetras too large for a 20? Who advising actually keeps these? I have a few that are several years old and I wouldn't hesitate to put a group of 4 or 5 in a 20L or even a 20H. As far as activity goes, they are actually pretty calm compared to danios and lots of the other tetra species.
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