How Many Cardinal Tetras in a 20 Gallon Tank?
About 8–11 — based on adult size, bioload, and real water volume.
In a 20-gallon tank you can comfortably keep about 8–11 cardinal tetra — enough for a proper school, on their own with good filtration and regular water changes. Add fewer if you're mixing in other species.
How we get that number
An adult cardinal tetra reaches about 2.0" and has a low bioload. Working from real water volume and waste output (not the old one-inch-per-gallon myth), a 20-gallon tank supports roughly 8–11 of them. Because cardinal tetra are schooling fish, keep them in a group of at least 6 — never just one or two. They need 73–81°F water at pH 5.5–7.
Build your exact stocking with the calculator → Full cardinal tetra care guide →
Gear for a 20-gallon tank
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