Your aquarium at the moment is overstocked. You need to consider finding new homes for the bala sharks and the clown loaches. Both bala sharks and clown loaches need a ton of space to roam around in and play and a 90 gallon just does not cut it. Sorry. Balas need at least 180 gallons and the same holds true for the clown loaches.
As far as the discus is concerned, a single discus is a bad idea. They are schooling fish and need to be kept in groups of no less than five. More if possible is the better option. Just having them in the 90 will max you out without considering your other fish. You also have fish listed in your sig. that are not considered compatible with discus such as gourmais which will be nippy to discus and rainbow fish that will also be nippy and aggressive to discus. The swortails will not be able to handle the high temps. that discus require. The only fish I see that you have listed that might work with discus are the angelfish and the bristlenose pleco. If you want discus you will have to revamp your entire tank.
Discus are a very social fish and like the company of each other. Usually a group five or more will be ideal. Due to territorial issues, keeping a few of them is harder then keeping a group of them in smaller tanks.
Also the angelfish can be carriers of discus plague, which do not harm the angels but can kill the discus. If you wanted, you could switch out the angels(and maybe some other group of fish) for a group of five to seven discus.