The three species of rummynose will almost always be together. This is one of the tightest shoaling characins. On the down side, they need a larger group (and thus a larger tank than some tetra) to be at their best, since they do like to swim. I have had a group of around 20 in my 5-foot tank for several years now, and they still swim end to end in strict formation quite often. Another point, they need soft slightly acidic water. They are wild caught and will not be as colourful or as long-lived in harder water. Hemigrammus bleheri is the more colourful species and the one most often seen in stores. H. rhodostomus is rarely seen any more, it is not as "brilliant" in the red colouration. Petitella georgiae is also less colourful, but still interesting as it is slightly larger and the tail is a stronger black/white pattern. In our profile of H. bleheri there is a chart showing the differences between these three species.
The species in the rosy clade of Hyphessobrycon also tend to remain close to each other, though these rarely "swim" to the extent of the rummynose but prefer just hovering around under the cover (shade) of plants, and away from any currents from the filter. But they do stay together nicely, and the frequent displaying of males is a beautiful sight.