A cardinal tetra carries its red stripe the full length of the body, from nose to tail, which is the quickest way to tell it from a neon tetra whose red stops halfway. Cardinals come from the blackwater tributaries of the Rio Negro, grow to barely an inch and a quarter, and are among the most peaceful fish in the hobby. Nothing about them defends itself: they do not nip, they do not compete hard for food, and their whole strategy for safety is a large school drifting over dark water in dim light.
The water writes the guest list
Before temperament even comes into it, the water narrows the choices sharply. Cardinals thrive between 73 and 81°F in soft water that leans acidic, ideally with a pH under 7.0, and they colour up fully only when those conditions hold steady. Hard-water livebearers such as platies and mollies, which prefer alkaline conditions, are therefore a poor fit even though their temperaments are fine. Every candidate below belongs on the list partly because it genuinely wants the same warm, soft, tea-tinted water, not merely because it can survive a few weeks of it.
Companions that share a cardinal's world
Neon tetras mix with cardinals so naturally that mixed schools often form, and rummy-nose tetras add a slightly larger, tight-schooling presence in the same water layer. Ember tetras and chili rasboras bring equally tiny, peaceful company. On the bottom, pygmy and hastatus corydoras sift the sand without disturbing anyone, while otocinclus graze algae from leaves and glass. Cherry shrimp colonies thrive alongside cardinals because the fish are too small and uninterested to hunt adults. In a large, warm tank, discus are the classic cardinal partner, and a pair of German blue rams or apistogramma can work where there is planting to give everyone their own ground.
Fish that turn cardinals into food or shadows
Angelfish are the trap: beautiful, commonly sold small, and natural tetra predators that will quietly eat cardinals once they grow. Any larger cichlid, predatory catfish or big-mouthed fish does the same arithmetic. Tiger barbs and similarly boisterous schoolers will not eat cardinals, but the constant darting and fin-testing keeps them pressed into the plants, pale and refusing food. Fast, greedy feeders are a subtler problem, because cardinals hang back at mealtimes and slowly lose condition in a tank where every flake is gone in seconds.
Helping a shy school feel safe
Keep ten or more cardinals so the school reads as one confident organism rather than a scatter of targets. Dense planting, floating plants to dim the light, and a dark substrate reproduce the shaded blackwater they evolved in, and their red deepens within days. Gentle filtration matters, since a strong current exhausts small fish that evolved in near-still water. Quarantine newcomers before they join the tank, because cardinals are sensitive to the water-quality swings that follow a careless addition. When the next addition is still being considered, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.