You obviously don't get my point. Sure, one discus may fit in there and so may 3. I know breeders that keep breeding pairs in 20H tanks and this is perfectly fine, because they are a BREEDING PAIR and they don't fight with each other. However, you seem to be missing the point that it's not whether the fish will FIT into the tank, it's whether it will WORK. I can tell you right now and all experts would agree with me that discus should be kept in groups of 5-6. This is because discus form a hierarchy and there will always be a leader and the weakest discus. With two you have one powerful and one weak. With three you have two picking on one. With four you have the aggression spread out a little more but it's still not a good number. With five you are at the minimum of a school size for discus to work for aggression to be spread out enough for all to be healthy. A single discus is not a good idea because it will just sit in the tank and will be stressed wondering where all of the other discus are. It will feel lost and will most likely become ill in a matter of a couple weeks. They need the socialization in order to survive.
So, once again, this is not about SIZE of the tank. I'm guessing your tank is roughly 30 inches long...not bigger than I think. I know how large tanks are, I have many 10's, a couple of 29's, a 55 and a 5ft 100g. I am very familiar with what will work in tanks but if you'd like to ignore my advice and get a single discus thinking that you knwo more than me and countless experts please go ahead and do so. Don't be surprised when I post an "I told you so" comment on your future thread. I'll offer you no further advice in that thread seeing as how you have completely disregarded any useful information provided in this one.