Tank Mates
Some fish keepers will house their Discus in a species only set up without, or just a few area selected species of tank mates, however in a domestic living room people like to keep what catches their eye in the shop rather than a dedicate biotope aquarium.
It is very important when selecting companion species for you Discus that you do not compromise the Discus's water requirements at all, so select only species that will be happy with a water temperature of 28-30 C and will tolerate your local pH and hardness etc.
Fish that are not too boisterous, and fish that will not hog all the food are those to consider - a prudent aquarist will research their chosen species before buying them and the Internet is ideal for this, so coupled with the advice of your retailer, getting it "wrong" is minimised. Remember also that tank mates should undergo a period of quarantine and observation prior to mixing with your Discus - many folk will not do this and have no issues, but you must be aware that bacterial compatibility issues are a very real risk.