About 5–7 — based on adult size, bioload, and real water volume.

In a 10-gallon tank you can comfortably keep about 5–7 neon 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.
An adult neon tetra reaches about 1.5" and has a low bioload. Working from real water volume and waste output (not the old one-inch-per-gallon myth), a 10-gallon tank supports roughly 5–7 of them. Because neon tetra are schooling fish, keep them in a group of at least 6 — never just one or two. They need 70–81°F water at pH 6–7.
Build your exact stocking with the calculator → Full neon tetra care guide →
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