Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
A 120l tank is too small for keeping 5 discus.
A 60 gallon tank (approx.225l) would be a proper size for keeping 5 or 6 discus to maturity.
Discus are not at all hard to keep, provided you get good quality, healthy, well-shaped stock from a reputable, experienced source to begin with.
Be careful of tankmates for discus - some of your current fish would not likely be suitable tankmates - e.g. the Juwel Rio, the flying foxes - perhaps even the guppies ( possible snacks for the discus), and the Angels (careful here - they can tend to potentially cross-contaminate discus with harmful pathogens, or be too aggressive, out-competing them for food.)