Why the number changes the fish
Some fish merely tolerate company; rummy nose tetras are built around it. In the wild they live in large, moving groups, and their whole security system is the school — many eyes watching, many bodies confusing a predator. Remove the crowd and a rummy nose does not become a solitary version of itself; it becomes a frightened one. The signals are easy to read once you know them: the red face that gives the fish its name washes out to pale pink, the school scatters into the plants instead of travelling, and feeding turns hesitant. Six is the widely used minimum because it is roughly where schooling behaviour switches on; the change from six to twelve is just as visible again.
What a good-sized school looks like day to day
In a group of ten or more, rummy noses do the thing people buy them for: they hold a tight formation and patrol the length of the tank together, wheeling at each end, the red faces and black-and-white striped tails moving as one ribbon of fish. Aquarists often remark that they school more tightly than almost any other tetra, which makes them a living barometer of the tank — a school moving calmly and coloured up fully is a school that feels safe, eats well and lives in water that suits it. That is why the group size is not a stocking technicality: it is the difference between keeping the species' actual behaviour and keeping a row of hiding individuals.
Making room: tank shape and stocking order
Because the school travels, length matters more than depth — a longer tank lets the group make its runs instead of circling a small space. Warm, soft, slightly acidic water suits them, and they colour best against planting and a darker substrate rather than bare bright gravel. Buy the whole group at once if you can, from the same tank in the shop, so quarantine and settling happen once and the school forms immediately; adding two fish every few weeks means months in which nobody has the right number. Stock them before bolder species claim the tank, and keep the total stocking honest — a big school of rummy noses plus a modest community is better than three small groups of three different tetras, all of them below their comfortable numbers.
Adding to the school later
Schools shrink over the years as individual fish age, and topping back up is part of keeping this species well. New arrivals should be quarantined if you possibly can, then added in a small group rather than singly — a lone new rummy nose dropped into an established school has a hard first week, while three or four newcomers settle together quickly. Feed a little extra for the first days and watch that the new fish are eating, because the established school will beat shy arrivals to the food at first. Keep the running total written down somewhere the whole household can see it. Access to the profile for the tank can be passed to a parent or grandparent you select by way of KinsLe Family Sharing.