Cardinal tetras are almost always bought as a group and should be sized as one. A single cardinal is a small, slender fish, but the creature you are really keeping is the school: a loose, glittering formation that drifts, turns, and feeds together, and that only forms properly when the fish are numerous enough and the tank long enough for movement to matter.
The size that lets a school behave like one
About 20 gallons in a long format is the sensible home for a school of ten or more cardinals, and it is the point where their behaviour changes from hovering near plants to travelling the tank in a body. A smaller group in a smaller tank survives, but the fish stay paler and tighter to cover, and every loss shrinks the school below the size where individuals feel safe. If you are choosing between a taller tank and a longer one at the same volume, choose the length every time; cardinals use horizontal distance constantly and the upper corners of a tall tank hardly at all.
Plan the school as the stocking decision, not the individual fish. Buy the group together, quarantine new arrivals where you can, and replace losses promptly once the cause is understood, because a dwindling school is a welfare problem for the fish that remain, not just an aesthetic one.
Layout inside those gallons
Cardinals come from slow, shaded, leaf-stained water, and the layout should soften the tank accordingly: plants massed at the back and ends, driftwood overhead in places, floating cover dimming part of the surface, and an open middle lane for the school's travelling. Their red stripe runs the full body length, and it shows best against dark substrate and greenery rather than pale gravel and bright bare glass. Gentle flow suits them; a school pinned against one end by a strong current is telling you the layout fights their nature.
Because they are sensitive to poor and unstable water, the larger volume earns its keep a second way, buffering the small swings that a tiny tank amplifies. Stable warmth matters more than chasing exact numbers, and changes, whether water, light, or decor, should come gradually.
Who shares the space
Choose tankmates that leave the school's layer and pace alone: peaceful fish of similar size or gentle bottom dwellers below them. Large or boisterous fish turn cardinals into permanent hiding fish no matter how big the tank is, and fin-nippers unravel the calm the school depends on. Feed small foods in the open lane and watch the whole group rise together, the simplest daily confirmation that numbers, space, and layout are right.
Their colour is the layout's report card. Cardinals holding a deep, unbroken red through the whole body in the middle of the day are telling you the school is big enough, the light is soft enough, and the water feels stable; fish that only colour up at feeding time are asking for more cover or more company.
Count the school weekly and treat the count as part of the tank's specification, as real as its length. Writing that target school size and the tank dimensions into KinsLe Family Sharing means a family member replacing lost fish buys enough to restore the group, not just a pair.