The short answer for a rummy-nose tetra
Give a school of rummy-nose tetras at least a 20-gallon long tank, and aim for a 30-gallon or larger aquarium if you want the group of twelve or more that this species deserves. Rummy-noses are celebrated for schooling more tightly than almost any other tetra, moving as one flashing unit with scarlet heads and striped tails. That behaviour needs a runway. In a short tank the school reaches the end glass every few seconds, breaks formation, and spends its life turning; in a long tank it settles into the rhythmic, end-to-end cruising that makes a planted display look alive.
Why their colour is a water and space report card
The red on a rummy-nose's head is famously honest. In clean, stable, suitable water in a settled school, the colour runs deep and bright; when conditions slip or the fish feel exposed, noses fade to pale pink within hours. Numbers and space are part of that equation. A small group in a small tank stays skittish, hugging the plants and paling whenever anyone approaches. A large school in a long tank grows bold, uses the open water, and holds its colour, which is why experienced keepers treat tank size and school size as one purchase, not two.
They come from soft, warm blackwater-influenced rivers, and while tank-bred fish are adaptable, stability matters more than any single target value. Frequent modest water changes in a mature tank suit them better than irregular large corrections.
Arranging a long tank around the school's lane
Plant the back and ends with stems and broad-leaved plants, keep a clear swimming corridor along the front and middle, and use a darker substrate and background to set off the red heads. Gentle, even flow distributed along the tank keeps the school oriented without exhausting them. Feed small foods in the lane so the whole group eats together, and watch for any individual hanging back in the plants, because in a species this synchronised, the fish out of formation is your earliest health warning.
Companions should share their calm: other peaceful tetras, small rasboras, corydoras-type bottom fish, and dwarf cichlids of a placid nature all work. Large, fast, or nippy fish collapse the schooling into hiding, whatever the tank size.
When a rummy-nose school needs more room
Faded noses in otherwise clean water, a school that startles as a single panicked ball at every room movement, and fish that refuse the open lane are environmental complaints. Usually the fix is more length, more companions, or both, in that order. Resist topping up a small tank with extra fish to solve shyness; that trades a space problem for a water-quality one. Step the enclosure up first, then let the school grow into it. Once the display is running, KinsLe Family Sharing keeps the headcount, the change schedule, and the pale-nose warning in the shared profile so any family member can spot trouble early.