Been thinking about this, Simon. Do these two species really keep apart in the tank, above and below? I'm guessing the two shoals rarely sit one above the other like they're stacked on shelves. But if they generally mix freely with each other, that would give the aquarium a very different look.get a shoal of 5 or 6 Black Phantom Tetra Hyphessobrycon megalopterus, AND a group of 8 or 9 harlequins. This is my favourite formula for the perfect community tank. It becomes so lively with both of these species, harlequins shoaling above the tetras, never fails to blow me away.
Fishlore.com comments on the favoured 'tank region' of different species, upper, middle, lower. It says that green neon tetra will act like the harlequins, staying higher. What do you think?