The fully grown measurement
A cardinal tetra finishes growing at roughly one and a quarter to two inches long, and most well-kept adults measure close to an inch and a half. It is a slim, torpedo-shaped fish, so even a maximum-size adult is a slight creature, with the famous colour doing the visual heavy lifting: an electric blue stripe running the full length of the body above a deep red that, unlike its neon tetra cousin, extends along the whole underside rather than stopping halfway. Females at full size carry a marginally deeper, rounder belly, especially when well conditioned, while males stay a touch sleeker, but nobody buys this species for individual bulk. What you are really acquiring is the combined effect of many small fish moving as one sheet of blue and red.
How long the growing takes, and what influences it
Young cardinals sold in shops are often barely an inch long and still filling out. In warm, soft, stable water with regular small meals, they reach near-adult length within several months and then settle, adding condition rather than length. Growth comes out uneven when the basics wobble: a school kept too cool slows down, one fed only large, infrequent meals competes unevenly so the boldest fish thrive while the shyest stay small, and fish raised in a crowded, high-waste tank rarely match the size of the same species raised with space and clean water.
What adult size means for the tank you choose
An inch-and-a-half fish sounds like it belongs in the smallest tanks, and that is the trap with cardinals. The individual is tiny, but the school is the unit: these fish look and behave properly in groups of six at the very least, and far better in double figures, where the group stretches, turns, and flashes together. A long twenty-gallon tank gives a proper school the swimming line it wants and the water stability this warm-water species depends on, while a small cube leaves the group bunched and nervous however few inches each fish measures. Filtration and heating sized generously for the volume matter more than squeezing the school into the smallest possible footprint, because a stable tank grows bigger, brighter adults than a cramped one ever will.
Full-grown cardinals among other fish
Adult size cuts both ways in a community. A two-inch cardinal is large enough to live safely beside other small, peaceful species, yet still small enough to be eaten by anything with a genuinely big mouth, so tankmates should be chosen by their adult size, not the size they were in the shop. It is also small enough to keep in numbers alongside bottom-dwellers and a calm centrepiece without the tank feeling stocked to the ceiling, provided the total group is planned rather than accumulated impulse by impulse. At feeding time, full-grown cardinals are quick and confident in mid-water, but tiny foods suit their small mouths better than chunky fare, and scattering food along the school's length stops the front row monopolising every meal.
Judging a school you are about to buy
In the shop, look for cardinals already showing full, unbroken colour, because a school at or near adult size with a solid red underside and a bright blue line has been living in water that suits it. Buy the group together, acclimate them slowly into warm, dim, planted surroundings, and expect their colour, more than their length, to keep improving for weeks as they settle. Years later, if that mature school is ever rehomed as a group, KinsLe Transferrable Profiles keep its headcount, feeding routine, and water history attached to the fish themselves, so nothing the new keeper needs is lost in the move.