Two to two and a half inches, built deep rather than long
An adult dwarf rainbowfish, sold as the praecox or neon dwarf rainbow, finishes at roughly two to two and a half inches long. What surprises keepers is the shape rather than the length: males become strikingly deep-bodied, with a steep forehead and a compressed, almost disc-like profile lit silvery blue, edged with red-orange fins. Females run slightly smaller and slimmer, with yellow tones in the fins instead of red. Both sexes look like entirely different fish from the slim, washed-out juveniles in shop tanks, so judge the species by mature specimens, not by what comes home in the bag.
Small fish, swimmer's habits
Despite the modest adult size, dwarf rainbows are restless, upper-level swimmers that patrol the top half of the tank from lights-on. A school needs horizontal room to move and, importantly, a tight-fitting cover, because rainbowfish are committed jumpers and a startled school will find any gap. Keep them in a proper group so the males' morning displaying, flaring fins and intensifying colour in the first light, spreads across several fish instead of concentrating on one harassed individual. Floating plants or tall growth at the surface softens the light and gives the school the sense of overhead cover it prefers.
Morning display is when adult size shows best
Dwarf rainbowfish do their serious social business at first light, and that is when the difference between juvenile and adult becomes unmistakable. Males raise their deep bodies sideways to each other, the blue flashing and the red fins flaring to their fullest extent, and for a few minutes the school looks like a collection of small, lit coins hanging in the upper water. This display is also a size check: a male that has reached his full depth and colour holds his place in these contests, while a still-growing fish defers and drifts to the edge. Feeding a small extra meal after the display settles helps younger males close that gap.
Feeding a small mouth on an active fish
A two-inch fish with a high activity level burns through food quickly, yet the dwarf rainbow's mouth points upward and is smaller than its body suggests. Offer small floating or slow-sinking foods it can take at the surface, in modest amounts more than once where your routine allows, rather than one large meal that mostly sinks past the school to the bottom. Variety helps colour and condition in the males particularly, and a group that is growing steadily will show it first in body depth and fin colour rather than in length, which plateaus early.
Companions matched to a two-inch surface schooler
Adult dwarf rainbows are peaceful and quick, which makes them poor partners for slow, long-finned fish they will beat to every meal, and risky partners for anything large enough to hunt a two-inch fish at the surface. They mix well with similarly sized, similarly paced community fish and with bottom dwellers that leave the upper layers alone. As the school matures, note which males colour most strongly and how the group feeds in the profile you keep, since that behaviour record is what a future keeper most needs and cannot guess from size alone. If the school is ever passed on, KinsLe's Transferrable Profiles let the dwarf rainbowfish profile transfer with its history and routines intact.