Tetras are a diverse collective of small, schooling freshwater fish belonging to the order Characiformes. While the name “tetra” is broadly used in the aquarium hobby, the vast majority of popular species originate from South America, where they are a dominant part of the freshwater ecosystems. These fish are characterized by their compressed bodies and the presence of a small, fleshy adipose fin located between the dorsal and caudal fins.
The Main Origin: South America’s Diverse Waters
The Neotropical region, extending from Central America down to northern Patagonia, serves as the evolutionary center for most tetras. The Amazon Basin and the Orinoco River system host the highest concentration and diversity of these small characins. This immense drainage area spans multiple countries, including Brazil, Colombia, Peru, and Venezuela, making it the primary source for the global aquarium trade.
Iconic species, such as the Cardinal Tetra (Paracheirodon axelrodi), are endemic to the upper reaches of the Rio Negro and the Orinoco drainage, thriving in slow-moving tributaries. The Black Skirt Tetra (Gymnocorymbus ternetzi) has a wider distribution, found across parts of Brazil, Argentina, and Paraguay.
The vast network of rivers and tributaries provides distinct niches, ensuring that some species are confined to specific areas of a single country. This localized distribution contributes to the ongoing discovery of new miniature species within the heart of the Amazon Basin. The Orinoco, for example, is home to over 400 species of characins alone, making them the largest group of fish in that drainage.
Habitat Characteristics and Water Chemistry
Tetras inhabit environments that can be broadly categorized into three distinct water types, each with a unique chemical profile. Blackwater habitats, such as the Rio Negro, are stained a dark tea color by humic acids and tannins released from decaying leaf litter and wood. This organic material creates highly acidic water, often with a pH ranging from 4.0 to 6.5, and very low mineral content, referred to as soft water.
This low-mineral, acidic environment is what many of the most recognizable tetras, including the Cardinal and Neon Tetras, have evolved to prefer. The tannins in the water act as a natural antibacterial and antifungal agent, which contributes to the fish’s health and encourages more vibrant coloration. In contrast, clearwater environments, often fed by mountain streams, have more neutral pH values and low levels of suspended sediment.
Whitewater rivers, such as the main channel of the Amazon, are turbid due to suspended clay and silt eroded from the Andes mountains. These waters carry more dissolved solids, resulting in a higher mineral content and a more neutral to slightly alkaline pH compared to the blackwater tributaries. While tetras are found across all these water types, their specific adaptations to soft, acidic conditions explain why many species thrive best when these parameters are replicated in captivity.
The African Exceptions
While the majority of species commonly referred to as “tetras” are native to South America, the family Alestidae represents a significant group found exclusively in Africa. These species are often called African tetras and include the popular Congo Tetra (Phenacogrammus interruptus). They are part of the same order, Characiformes, but their evolutionary path diverged from their South American counterparts millions of years ago.
African tetras are primarily distributed across the central African waterways, most notably the expansive Congo River Basin. The Congo Tetra is endemic to this region, inhabiting streams and marshes where the water is typically murky and slightly acidic. Although they share the characteristic adipose fin with South American tetras, their existence highlights that the “tetra” designation broadly applies to similar fish across two continents.