Two to two and a half inches, red at one end, striped at the other
An adult rummy nose tetra finishes at about two to two and a half inches long, slim and silver through the body, with the colour concentrated dramatically at the ends: a saturated red head and a tail fin striped in black and white like a warning flag. Sexes are hard to separate by size, females running only slightly fuller when carrying eggs. Because several similar species are sold under the rummy nose name, mature size varies a little between them, but all stay within this small range and share the same schooling character that defines the fish in a home tank.
The tightest school of any tetra this size
Rummy noses are kept for one behaviour above all: they school in a tight, disciplined formation, the whole group turning on the same instant and travelling the tank as a single organism. That behaviour only appears in a group of real size, in water stable enough for the fish to relax; a few individuals in a bare tank drift apart and look like any small silver tetra. Give the school a long, planted tank with open midwater to parade through, and their formation becomes the centrepiece, red heads flashing in unison with every change of direction.
Three fishes wear the rummy nose name
The red-headed, striped-tailed fish sold as rummy noses are not all the same species, and the differences show at adult size. The true rummy nose carries its red furthest, often past the gills, and reaches the upper end of the size range, while its two lookalikes finish slightly smaller with the red confined more tightly to the head. All three school together without distinction, and mixed shop groups are common and harmless. For planning purposes the distinction barely matters, since housing, grouping, and companions are identical, but it explains why one school may look redder or grow a fraction larger than another keeper's.
Reading the red nose as a daily health report
Uniquely among the tetras in this size range, the rummy nose wears its condition on its face. The red is full and deep when the school is settled, well fed, and comfortable in its water, and it drains to a pale pink within hours when conditions slip, after transport, or when a fish is unwell. Experienced keepers glance at noses before anything else during a morning check. Feeding supports that colour: small foods taken in midwater, offered to the moving school rather than dumped in one corner, with enough variety that the whole group, front rank and rear, eats at every meal.
Companions for a sensitive, mid-sized schooler
Rummies are peaceful but more delicate than their size suggests, suited to calm, similarly sized companions in stable water and poorly matched with boisterous fish, big fin-nippers, or anything that would view a two-inch tetra as food. They reward tanks that are already mature and steady rather than newly set up. Record the school's size and how their colour responds in your setup in the profile you keep for them, because that living baseline, what normal red looks like for this group, is the most useful thing a future keeper can inherit. If the school moves on, KinsLe's Transferrable Profiles let the rummy nose tetras' profile transfer to its new keeper with their history and routines intact.