Can cardinal tetras live with black neon tetras?

Verdict
Yes

Yes. Cardinal tetras and black neon tetras make a good pair. Both are fine at 76–81°F. Plan on at least 20 gal.

Cardinal tetraCardinal tetra76–84°F · pH 4.6–7 Black neon tetraBlack neon tetra73–81°F · pH 5.5–7.5

Why it works

Water side by side

55°60°65°70°75°80°85°90°Both: 76–81°Cardinal tetra 76–84°Black neon tetra 73–81°
The green band is the temperature range both can live in.
456789Both: 5.5–7Cardinal tetra 4.6–7Black neon tetra 5.5–7.5
The green band is the pH both tolerate.

Size to scale

Cardinal tetra · 2 inBlack neon tetra · 1.5 in0"1"2"3"4"5"6"
Adult length in an aquarium. A fish can swallow anything that fits in its mouth, roughly a third to half its own length for predators.

Care at a glance

Cardinal tetraBlack neon tetra
Temperature76–84 °F73–81 °F
pH4.6–75.5–7.5
Adult size2.0 in1.5 in
Minimum tank20 gal15 gal
TemperamentPeacefulPeaceful
Swims atMiddleMiddle
GroupGroups of 6+Groups of 6+
DietOmnivoreOmnivore

Better tank mates for cardinal tetras

All cardinal tetra tank mates

Better tank mates for black neon tetras

All black neon tetra tank mates

Questions

Can cardinal tetras live with black neon tetras?

Yes. Cardinal tetras and black neon tetras make a good pair. Both are fine at 76–81°F. Plan on at least 20 gal.

What temperature suits both cardinal tetras and black neon tetras?

76–81 °F. Cardinal tetras need 76–84 °F, black neon tetras need 73–81 °F.

What size tank do cardinal tetras and black neon tetras need together?

At least 20 gal, set by the cardinal tetra. Add space for each extra fish.