Interested to know why? Or the mix of them with other species?
Discus are slow eaters. They like to graze. Almost all medium to large SA cichlids will outcompete them for food every time.
If people are not familiar with discus, they should always start with a species-only tank and learn all the tricks and quirks to discus before trying to mix them with other species, especially bold and boisterous species like Guianacara, Geophagus, etc.