Can discus live in tap water?

Provided the tap water parameters are those that are required for keeping Discus there should be no problems. You would of course have to make sure any chlorine or chloramine is neutralised. Parameters for Discus are pH about 5.8 to 6.2 Total hardness about 25ppm but should be <100ppm. Temperature around 82F to 87F (never below 80F). Minimum size tank for Discus (they are shoaling fish and when young need to be in groups of 5 or more) is 48"x 18"x18" with a well cycled power filter to suit tank. Discus need a minimum of 10% water changed daily and at least 50% changed weekly. The tank can be bare for ease of cleaning but some people take the risk and have a substrate and plants.