Your fish probably trace back to one of four river basins

Every January, the town of Barcelos on Brazil's Rio Negro holds a festival for a fish about two inches long. Fishers there net small fish from the river's forest streams for the aquarium trade, and roughly 80% of what they sell is a single species, the cardinal tetra. Project Piaba, a conservation effort that began on the river around 1990, has long argued that the fishery gives local communities a reason to keep the forest standing. Its slogan is “Buy a fish, save a tree.”

The Rio Negro feeds the Amazon, which holds more fish species than any other river system, 2,406 native species by one 2020 count. It is also the river that turns up most in members' aquariums. A Shoal & Stem analysis traced the fish in each member tank to the rivers where they live in the wild, and just over half of the tanks with fish hold at least one species native to the Amazon basin.

Three other rivers are not far behind: the Orinoco, the Amazon's northern neighbor, and the Mekong and the Chao Phraya in Southeast Asia. More than 8 in 10 member tanks with fish hold at least one species from one of the four.

The four rivers

A river basin is a river and every stream that drains into it, so a basin's fish include those of its smallest forest creeks. These are the four, with the share of tanks that hold their fish and the species members keep most from each.

Not every favorite comes from the four. The ember tetra is native to Brazil's Tocantins, the cherry barb to rivers in Sri Lanka, the honey gourami to the Ganges, and platies and mollies to Mexico and Central America.

The rivers come in pairs. In southern Venezuela a channel about 220 miles long, the Casiquiare, carries a fifth to nearly a third of the upper Orinoco's water into the Rio Negro, joining the two river systems. Three quarters of the Orinoco fish in the Shoal & Stem catalog also live in the Amazon. The cardinal tetra is one of them: the fish of Barcelos is native to both. The Mekong and the Chao Phraya overlap the same way, and nearly every Chao Phraya fish in the catalog also lives in the Mekong.

Thailand's fighting fish

The most familiar fish from the Mekong and the Chao Phraya is the betta. Wild bettas live in shallow, nearly still water, such as marshes, floodplains and rice paddies, in both basins. In Siam, now Thailand, males were caught and bred for fighting, and by the 19th century the fights were popular enough that King Rama III taxed them. The British ichthyologist Charles Tate Regan gave the species its name, Betta splendens, in the early 1900s. In the wild it is now listed as vulnerable, while bettas bred for the trade live in about 1 in 4 member tanks with fish.

Rivers that never meet

In the wild, a cardinal tetra from the Rio Negro and a betta from a Thai rice paddy live on opposite sides of the world. In member tanks, fish from those two worlds often share the same water. Of the tanks with at least two fish whose native basins are known, 63% hold fish from both tropical America and tropical Asia.

The few that keep to fish from a single river are the kind of tank biotope keepers set out to build, a copy of one river's community.

One tank, one water

Fish from different rivers in one tank have to share one water. The Shoal & Stem catalog lists a typical Amazon fish for a pH of about 6.0 to 7.5, and members' tanks with Amazon fish sit toward the top of that range.

Their latest readings have a median of 7.2. About two thirds of the Amazon fish members keep live in water inside their listed pH range, and the rest live in water above it; none live in water below it. Hardness fits more closely: 3 in 4 of those fish live inside their listed GH range.

Back to Barcelos

Many of the fish on Shoal & Stem have never seen the rivers they trace back to. Most freshwater aquarium fish sold today are bred on farms, and the neon tetras sold in the United States come mainly from farms in Southeast Asia. Thailand breeds bettas for buyers around the world, and Nakhon Pathom province, just west of Bangkok, has more betta breeders than any other part of the country.

The cardinal tetra is one of the exceptions. Breeders raise it now too, but a large share of the cardinals sold are still netted in the wild, in the upper Orinoco and in the forest streams around Barcelos, and Project Piaba's fish-for-a-tree bargain depends on that catch. On Shoal & Stem the neon tetra is the more common of the two, in about 1 in 14 tanks with fish, while cardinals live in fewer than 1 in 20.

By Project Piaba's account, most of the cash income in Barcelos has long come from catching small fish for aquariums. For the festival each January, the town is decorated in the colors of its two rival teams, and one team's colors are the cardinal tetra's red and blue.