We use tap in our discus tank, but I'd say buy from a local breeder (Hans) if you want Discus that'll do well in local tap.
Discus need space, numbers, dither fish, and clean water. Like many fish, they pick on each other and a larger group helps thin out the impact of aggression. Buy juvies not because they're cheap, but because they haven't been speed-grown on beef heart (can cause problems like digestive issues and fatty liver). People like to speed grow fish and sell them for top dollar and you don't realize you're buying the piscine equivalent of Chris Farley (RIP).
A good aquascape will help with aggression and stress, plus makes your fish happier (scientists have actually researched the impact of a decorated vs sparse environment on the development of fry). Even in a 150 gallon we have lost fish due to aggression. The weakest few eventually go from stress, picking, and related anorexia.
That said, your tank really isn't big enough because of its length. You could do a pair and a bunch of schooling fish if you knew someone parting with a healthy couple, but a group is stretching it.
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