The temperament rule: calm and similar-sized
Rummy nose tetras are small, nervous, warm-water fish, and their tankmate rule follows from those three facts. Anything calm that cannot fit a tetra in its mouth and that enjoys the same warm, soft water is a candidate; anything boisterous, nippy or predatory is not, however pretty the combination looks in the shop. Match the water first — rummy noses do best warm, and a companion that prefers cool water forces one species to live at the edge of its range. Then match the energy: a rummy nose school holds together beautifully in a quiet tank and spends its days hiding in a loud one, and a hidden school loses colour, feeds poorly and stresses itself sick.
Companions that work, group by group
The tetras' own family is the easiest place to start: cardinal tetras and ember tetras share the water and the temperament, and mixed schools often loosen into one larger, relaxed group. Harlequin rasboras bring a slightly bolder mid-water presence without any edge. For the bottom, corydoras are the classic partner — peaceful, busy, and active at a level the tetras ignore — with otocinclus quietly clearing algae from leaves and glass, and kuhli loaches slipping through the substrate after dark. A single honey gourami can work as a gentle centrepiece where a larger gourami would not. Every one of these is itself a social fish, so stock each in its own proper group rather than as single specimens.
The tankmates to avoid
The classic mistake is the angelfish bought small: an adult angelfish's mouth and instincts are exactly sized for a tetra, and keepers lose rummy noses one at a time overnight. Any larger cichlid carries the same risk. At the other end of the scale, avoid the nippy and the frantic — tiger barbs are the famous fin-nippers, but any hyperactive fish that darts at the school all day does similar damage through stress alone. Also think twice about long-finned, slow fish in the other direction: a rummy nose school is peaceful, but a betta in warm community water can be outcompeted at feeding time and stressed by the school's constant movement, so that pairing depends heavily on tank size and the individual betta.
Building the community in the right order
Add the rummy nose school first and let it settle, colour up and learn the tank for a few weeks before companions arrive; a confident established school tolerates new arrivals far better than new arrivals tolerate each other. Add each companion group in one go, quarantined first if you can, and watch feeding for the first days — the question is always whether everybody eats, because the shyest fish at the bottom of the order is the one that quietly loses condition. Keep the final stocking list modest: a good school of rummy noses, one companion group in the middle and one on the bottom is a complete, calm community. With the list and quarantine dates kept on KinsLe's Family Sharing profile, anyone feeding or maintaining the tank can see what lives in it and what has already been treated.