Orinoco Clearwater Biotope Aquarium

How clearwater differs from blackwater and whitewater, what the Orinoco basin looks like, its cichlid, tetra and catfish fauna, and how sand, rock, wood, light and flow recreate it.

Three kinds of South American river water

Lowland rivers of the Amazon and Orinoco basins are grouped into whitewater, blackwater and clearwater types by their chemistry, sediment load and colour. Whitewater rivers rise in the Andes and carry a heavy load of suspended clay, which gives them a pale, milky-coffee colour and a visibility of only about 20–60 cm. Their pH is near neutral (about 6.5–7) and their conductivity is high, 120–200 µS/cm near the mountains and 40–70 µS/cm further downstream, where blackwater and clearwater tributaries dilute them. In the Orinoco basin the Guaviare, Meta and Apure are whitewater rivers.

Blackwater rivers flow slowly through forested swamps and wetlands on nutrient-poor soils. Tannins and humic substances leached from decaying plant matter stain the water tea-brown. Measurements from the Rio Negro, the largest blackwater river, give a pH of about 5.1 ± 0.6 and a conductivity of about 9 ± 2 µS/cm. In the Orinoco basin the Atabapo and Inírida are classic blackwater rivers, and Wikipedia also lists the Caura and the Caroní, the largest Venezuelan tributary, as blackwater systems.

Clearwater rivers drain the ancient, heavily weathered crystalline rock of the Guiana Shield and the Brazilian Plateau. Because these Precambrian rocks release few dissolved solids, conductivity is very low: one comparison gives about 14 µS/cm for the Tapajós against about 191 µS/cm for a whitewater river. The pH is typically neutral to slightly acidic within a broad recorded range of 4.5–8, and the water is transparent with a greenish tint and a visibility of 1.5–4 m. The sediments carried are mainly sand and kaolinite. Wikipedia describes the Ventuari as the largest clearwater tributary of the Orinoco.

The Orinoco basin

The Orinoco is 2,140 km long and drains roughly 1,000,000 km², 65 % of it in Venezuela and 35 % in Colombia. With a discharge of about 39,000 m³/s at the delta it is the third-largest river in the world by volume, and it describes an arc around the Guiana Shield. The Casiquiare links the upper Orinoco to the Rio Negro and thus to the Amazon system. In the rainy season the main river can widen to 22 km and deepen to 100 m, a seasonal pulse that also inundates forest and grassland along the tributaries.

More than 1,000 fish species are recorded from the basin and about 15 % are endemic; the orders Characiformes (tetras and relatives) and Siluriformes (catfishes) together make up more than 80 % of the freshwater species. The Ventuari alone, a 520 km clearwater river with a 40,000 km² basin, had 470 known fish species as of 2006. Tropical Fish Hobbyist documents the Colombian Rio Guejar: fast, shallow water at about 22 °C in the dry season, fine grey sand under fist- to football-sized rocks, wedged driftwood and no aquatic plants.

Typical fauna

Cichlids

Pterophyllum altum occurs in the Inírida and Atabapo and in the upper Rio Negro. FishBase gives a maximum length of 18 cm TL, a temperature range of 27–31 °C, pH 4.8–6.2 and hardness of 1–5 dH, and lists the species as Least Concern on the IUCN Red List (2020; verify the current entry). Seriously Fish stresses that soft water with virtually undetectable hardness is mandatory, that the fish inhabit very slow-flowing rivers, and that they are highly sensitive to nitrate. Biotodoma wavrini, whose type locality is the lower Ventuari, is a sand-sifting eartheater reaching 10 cm SL that Seriously Fish keeps at 20–30 °C, pH 4.0–6.5 and 1–5 dGH. Apistogramma hongsloi comes from the Vichada, the middle Meta and the middle Orinoco, where it lives in small tributaries and still waters; it reaches 6 cm SL and is kept at 23–29 °C, pH 5.5–7.0 and 1–15 dGH.

Tetras and other characins

Paracheirodon axelrodi is native to the upper Orinoco and Rio Negro basins, including the Guaviare, Inírida, Meta and Vichada drainages. Seriously Fish notes that Orinoco populations often live in morichals, palm-lined streams with clearer water and sandy substrates, at a pH of roughly 5.5–6.5, unlike the very acidic Rio Negro populations. FishBase records a maximum size of 3 cm SL, a temperature range of 23–27 °C and Least Concern status (2021; verify the current entry), and describes shoals in the middle water layers feeding on worms and small crustaceans. Seriously Fish advises groups of at least 8–10.

Catfishes

Corydoras habrosus is recorded chiefly from the Apure watershed of the Venezuelan Llanos. It occupies lagoons, tributary streams, inundated forest and grassland where flow is weak, and aggregates near submerged vegetation, branches and roots. Seriously Fish gives 35 mm SL, 20–26 °C, pH 5.5–7.5 and 2–10 dGH, and recommends groups of at least 4–6, ideally 10 or more. Hypancistrus inspector, from the Casiquiare and upper Rio Negro, is collected in rocky rapids and reaches 16 cm SL; Seriously Fish keeps it at 22–30 °C, pH 5.0–7.5 and 1–15 dGH with caves, a moderate to high level of dissolved oxygen and some water movement. In the Rio Guejar the rocks are grazed by Chaetostoma joropo.

Sand, rock and wood

Sand is the defining substrate of clearwater rivers, and Tannin Aquatics recommends a fine grade of under 1 mm for sand-sifting cichlids such as Geophagus and its relatives. Seriously Fish likewise specifies a soft, sandy substrate for Biotodoma wavrini, Apistogramma hongsloi and Corydoras habrosus. Water-worn rocks and rounded boulders reproduce the rapids habitats of Hypancistrus and Chaetostoma and give Biotodoma spawning surfaces; branching driftwood and roots complete the structure. Tannin Aquatics advises against heavy leaf litter in a clearwater layout because the humic release would tint the water towards a blackwater look. Aquatic plants are scarce in fast, seasonally scoured sectors; for a calmer stream Seriously Fish lists Microsorum, Taxiphyllum, Cryptocoryne and Anubias.

Lighting and flow

Light reaches far deeper into clearwater than into blackwater, yet the fishes still come from shaded forest and palm streams. Seriously Fish recommends fairly dim lighting with floating plants for Apistogramma hongsloi, dim lighting for Paracheirodon axelrodi and Biotodoma wavrini, and shaded nooks for Hypancistrus. Flow depends on the sector chosen: the Rio Guejar is fast and shallow, Corydoras habrosus lagoons have weak flow, and Pterophyllum altum inhabits very slow rivers. For rapids species, filtration or a circulation pump should keep the water well oxygenated and moving; for Apistogramma hongsloi, by contrast, Seriously Fish advises gentle flow and small weekly water changes of 10–15 %.