Paracheirodon axelrodi, commonly known as the Cardinal Tetra, is a small freshwater fish native to the Orinoco and Negro River basins in South America. It is known for its vibrant red and blue coloration, social behavior, and preference for warm, acidic waters rich in tannins.

Paracheirodon axelrodi, commonly known as the cardinal tetra, is native to the upper Orinoco and Negro River basins in South America. This includes regions in both Colombia and Brazil.

The cardinal tetra inhabits slow-moving blackwater rivers and streams. These environments are characterized by clear, acidic water with a pH ranging from 4.0 to 6.5. The water temperature typically ranges from 23°C to 27°C (73°F to 81°F). The habitat is often shaded by dense tropical rainforest, which limits sunlight exposure and contributes to the tannin-rich, tea-colored water.

Within this habitat, cardinal tetras primarily occupy mid-water levels where they feed on small invertebrates, plant matter, and organic detritus. They play a role in the food web as both prey and predator, serving as a food source for larger fish and birds while controlling the population of smaller aquatic organisms.