Daylight fish, through and through
Rummy nose tetras are river fish built for the bright, open water of the school, and their whole daily pattern runs on the light. During the tank's lit hours they move as one body, cruising the middle layer in tight formation, breaking ranks to feed and reforming in a way that makes them a favourite for planted aquascapes. Their famous signal, the deep red nose, is a daytime display at full strength only when they are settled and conditions suit them. Expect the liveliest schooling not at the moment the lights snap on, but once the group has woken properly and the tank has settled into its day.
What the school does after dark
When the lights go out, rummy noses power down. The school loosens, individuals settle low among plants or hover nearly motionless in sheltered water, and the red drains from their faces to a pale pink-grey. New keepers, checking the tank last thing, often fear the worst at this washed-out stillness, but it is ordinary sleep behaviour and it reverses within a short while of the lights returning, as colour and formation rebuild together. A ramped or gradual light change suits them better than an instant switch in either direction, and a room light left on briefly after the tank lights go off softens the transition for a nervous new school.
Reading the nose as a daytime instrument
Because the colour that fades harmlessly at night should be blazing by day, the rummy nose gives you a built-in condition gauge, and daylight paleness is the reading that matters. A school that stays pale, breaks formation and hovers separately during lit hours is responding to something: new-tank instability, ammonia or nitrite traces, water too cool or too different from their soft, warm preference, harassment from a bigger tank mate, or simply too small a group to feel brave. Check the water first, then the company they are keeping, then group size; rummy noses school tightly precisely because numbers are safety, and a handful of them in a large tank will look and behave like a much unhappier fish than a proper school.
Arranging the tank around their rhythm
Give them a long swimming run for the daytime schooling, with planted or shaded margins they can retire into as evening comes, and keep the lighting period consistent from day to day so the school's clock stays set. Feed during full light when they are boldest and competing well, and site the tank away from heavy night-time disturbance, since repeated sudden room lights at midnight restart the startle-and-fade cycle. Tank mates that are active and pushy at night, particularly boisterous bottom dwellers that blunder through the resting school, cost rummy noses the quality of the rest their daytime display depends on.
When the pattern itself is the question
The useful observation with this species is rarely one dramatic event; it is a change in the daily rhythm, a school a little paler this week, a little slower to form up after lights-on. A change like that is worth putting into words while it is fresh, rather than reconstructing it from memory later. Put a follow-up question to KinsLe's Kimmy about the change; she answers using the KinsLe Answers library.