If a rio 180 is 180 litres - approx. 45 gallons - and if you consider that discus get to 8" if kept properly and should be in a group (only in the case of a breeding pair should they be housed in twos and I'd never keep one alone. 3 is also a bad idea in case 2 pair up so I would stick to even numbers - prefferably 6.), the tank isn't realy large enough for these very sensitive cichlids. If you could find a proven - adult - breeding pair, they would work but it's often hard to find a true breeding pair and you also said you're not interested in breeding. I wouldn't keep 6 discus in anything less than a 55 gallon. I suppose you could get 4 discus for the 45 but it's not ideal. There are plenty of other fish you could try instead. If you're after color, maybe look into some of the rainbowfish, larger 'fancy' livebearers (swords or mollies), gouramies (watch what you mix with what though), purple-spotted gudgeons (unusual and colorful) or some of the smaller neotropical dwarf cichlids - or maybe an african dwarf such as kribensis/pulcher. If all you want is that 'wow' factor, anything bright or unsual will do it if you are careful in what you select and plan out your aquascaping around them to compliment the tank's inhabitants.
cheers guys
