Cardinal tetras have shared tanks with ornamental shrimp for decades, and they earned that reputation honestly. A cardinal is a small, soft-water schooler whose mouth is built for insect larvae and tiny crustaceans at the surface film, not for hunting adult shrimp — a grown cherry shrimp is nearly the fish's own depth of body, far beyond anything it would attempt. The cardinals drift mid-water in their neon file, the shrimp graze the moss below, and each largely ignores the other, which is exactly what a mixed tank needs.
The one honest exception: the first days of a shrimplet's life
Newborn cherry shrimp are a few millimetres of near-transparent prey, and in open water any small fish, cardinals included, will take them opportunistically. This is a housing problem, not a compatibility failure. Give the colony a nursery the fish cannot patrol: a dense cushion of moss, a thicket of fine-leaved stems, cholla wood or a pile of small stones where babies spend their first vulnerable weeks grazing biofilm in cover. A breeding colony in a planted tank normally produces far more shrimplets than a school of cardinals can find, and the population grows regardless — what cover changes is how many of each generation make it. Introducing the shrimp first, or adding shrimplets when the cardinals are well fed, tilts the odds further. Stocking order helps in the other direction too: let the shrimp colony establish for a few weeks before the fish arrive, and it will already know its hiding places by heart. At feeding time the two crews barely compete — cardinals snatch food from the surface and mid-water within moments, while sinking shrimp food offered after lights-out reaches the colony undisturbed, when the tetras are resting and the shrimp come out to work the tank in the dark.
Conditions both sides already agree on
Cardinals want warm, soft, slightly acidic water and dim, planted surroundings; cherry and most ornamental shrimp tolerate that warmth comfortably and thrive among the same plants. Stability matters more than chasing textbook numbers — shrimp moult on a rhythm set by steady parameters, and sudden swings cost more shrimp than any tetra. One chemical warning belongs in every mixed tank: copper, present in many fish medications and some algae treatments, is lethal to shrimp and snails even at doses harmless to fish. Treat sick fish in a separate hospital tank, and check the label of anything before it goes into water that holds invertebrates.
Snails need no negotiation at all
Nerite, mystery and other common aquarium snails are simply not on a cardinal's menu: too big, too hard, too slow to register as food. Cardinals will not pick at snail eggs on the glass or bother a mystery snail's antennae, and the snails return the favour by grazing algae the fish leave alone. Keep the school at six or more cardinals so their confidence stays in the group rather than turning to nervous fin-nipping, record the tank's full cast — fish, shrimp colony, snails — in KinsLe's Family Sharing, and anyone who feeds or maintains this aquarium can see at a glance which creatures the water has to suit.