Most of the familiar fish kept by aquarists, from tetras to rasboras and from angelfish to many of the well-known catfish, are native to rivers across the tropics. Some of these rivers are huge -- the Amazon river alone holds within its banks an incredible 20% of all the worlds freshwater.The character of a river typically changes as it flows from its hilly origins to the wide, mature stretches that wind across the country to the sea. A river can also change throughout the year, flooding during the rains but slowing to a trickle during the dry season. River fishes, therefore, have to cope with a wide variety of conditions. The benefit to the aquarist is that they tend to be less demanding about their conditions in an aquarium.
What are the characteristics of fish that live in fast-flowing rivers?
In the upper reaches the river is fast-flowing and well-oxygenated, but this is a hostile environment for fish. Few can withstand the current and even for those that can, there is little food. Nonetheless, some fish do make their home here among the rapids, including the White Cloud Mountain minnows; the blockhead cichlid (Steatocranus), which seeks refuge and hunts among the boulders of the rivers of West Africa; and the Plecostomus catfish, which grazes on the film of algae covering the substrate, holding itself in the current with its powerful sucking mouth. Living in this harsh environment demands adaptations - most importantly a streamlined body shape.The noise of the tumbling waters and the turbulence mean that detecting pressure or faint odors in the water is difficult. At close quarters food may be sniffed out, but vision is of primary importance in these clear waters.
How do things change for fish as the river currents slow?
As the river moves downstream, the pace relents and life for fish is easier. Often, pools form along the course
of the river, offering a current-free haven for many species. Many of the familiar Central American cichlids occupy these reaches, breeding among the stones of the river bed, often in the larger pools out of the current. Here, streamlining is less important but even so, fish such as guppies occupy the water margins to avoid both the currents and the threat of predation. For most of the year the water is clear, allowing the fish to hunt and identify other fish by sight. As the water slows chemical cues become ever more important. Body
shape is more variable than in the upper reaches. Deep bodies allow maneuverability out of the current, whereas more torpedo-shaped profiles are common among those fish that live in the water current.
What kinds offish live in the lower reaches?
When the river reaches lowland plains, it widens and slows to a leisurely pace. In many of the larger rivers of the tropics, the water is colored by tannins and organic debris. The deeper waters allow little light to penetrate and the catfish that live along the bottom rely heavily on smell and taste to find food and to recognize others. The silty substrate harbors a rich array of invertebrates, and species such as the elephant-nosed fish probe for them in the darkness Nearer the surface and at the margins, fish such as cardinal tetras use their brilliant livery to signal to one another and keep to their schools.