it really depends. for just a 20 gallon tank i would go with 5 tetras (rummynose, neon, cardinal, glolight, black or white skirt, etc), a pearl gourami, and four corycats for the bottom.
if you decide that you want to keep discus, i'd suggest nothing smaller than a 75 gallon tank. the tank will have to be VERY well established (set up for months, aside from the cycle, before you can start adding discus) and have absolutely pristine water. discus keepers generally do water changes every day or every other day. i know Slug keeps discus, you may want to post a message on his page and ask him for more info
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