What fish is that?

The neon tetra is native to blackwater and clearwater streams in the Orinoco and Amazon basin of South America. Its bright colouring makes the fish visible to conspecifics in the dark blackwater streams, and is also the main reason for its popularity among tropical fish hobbyists. It is often confused with the cardinal tetra, but its red coloration extends only about halfway to the nose, and its blue stripe is a less vibrant blue.