While doing some unrelated research, I came across a study of discus ecology published by a biologist from the University of Florida. As it's a scientific paper, the language is technical. I excerpted some bits about discus diet, as that seems to be of great interest among forum members. Text follows:

The alimentary canal of Symphysodon is characterized by a poorly defined stomach and an elongate intestine, some 300 mm long and 3 mm wide (in a 180 mm SL specimen). This intestinal morphology is typical of a cichlid with a dominantly vegetarian, detritivorous, or omnivorous diet. Predominantly piscivorous cichlids such as Cichla and Crenicichla exhibit shorter alimentary canals with well developed stomachs (Zihler, 1982).

Bleher (2006, p. 510-595) reports detailed observational notes on the diet of discus, taken over many years of field visits to the Amazon basin. He undertook stomach content analyses on over 8,500 discus specimens and also made direct observations of feeding in the wild. Although most of his findings are reported qualitatively, Bleher (2006) presents some quantitative data for the volumetric dietary intake of S. haraldi (although numbers of specimens are not given, p. 593).

During the high-water period he reports average stomach contents of: 12% algae and microalgae, 44% plant matter (flowers, fruits, seeds, leaves), 6% detritus, 16% aquatic invertebrates, and 22% terrestrial and arboreal arthropods. During the low water period he reports 25% algae and microalgae, 39% detritus, 9% plant matter, 22% aquatic invertebrates, and 5% terrestrial and arboreal arthropods.

Data for S. aequifasciatus and S. discus indicate a larger proportion of algae, plant matter and detritus both for during the low and high water periods. The data presented here for S. haraldi from the Amană region indicate a pattern of lower dietary variability, and a much larger proportion of periphyton/detritus than reported by Bleher (2006). These discrepancies might reflect the small sample sizes reported here; perhaps much larger samples are required to show the true breadth of discus diet. Alternatively these discrepancies might reflect natural variation in diet among populations and species of discus.