Dragonfish do better in brackish water although it's said they can also be fine in freshwater. In a 55 gal
you could probably do 1 if you wanted other fish, two if you just wanted to keep them alone. As they grow though you may need to upgrade as they can reach a pretty good length, at least a foot.
Dragonfish Care Basics - The First Tank Guide - Basic Information on Keeping a Dragonfish in Your Aquarium
As for Discus, they are awesome fish but need some special care.