British ichthyologist William Crampton has provided the first detailed study of Discus ecology, publishing his results in a recent issue of the journal Neotropical Ichthyologist.
Crampton studied the feeding, reproduction and micro-habitat preferences of the Blue and Brown Discus in the vicinity of Tef, Brazil.
This species is known as Symphysodon aequifasciatus if you follow Kullander et al. and S. haraldi if you follow Bleher et al. (the paper uses the latter name for this species). (See Discus genus revised and New Discus named Symphysodon tarzoo)
The author found that the Discus feeds predominantly on algal periphyton, fine organic detritus, plant matter, and small aquatic invertebrates, with invertebrates making up a greater proportion of their diet during low water.
At high water, the Discus forages alone or in small groups in flooded forests; at low water, it forms large aggregations in fallen tree crowns (galhadas) along lake margins.
It is thought that the fish aggregate at galhadas as a form of protection against predators, which can reach very high densities during low water.
The discus appears to spawn in shallow shore scrub, in isolated pairs (the author could find no evidence that the Discus spawned in crowded galhadas). The bulk of Discus spawning occurred at the beginning of the rising water period (November"January).
For more information, see the paper: Crampton, WGR (2008) Ecology and life history of an Amazon floodplain cichlid: the discus fish Symphysodon (Perciformes: Cichlidae). Neotropical Ichthyology 6, pp. 599"612.