I kept a pair of Brown Discus in a 30 gallon planted tank. If I was to do it over the plants would be in clay pots and there would be a bare bottom. To clean the gravel as well as it needs to be cleaned is very hard on plant roots. My last Discus go around I had plants in clay pots and a bare bottom tank. My 75-90% daily water change made the plants and fish grow extremely well. I could also use plant ferts in the pot and not disturb the fertilizer buy sucking it out cleaning the gravel. Less than a 30gal you'll need to choose plants or fish as I personally thinking 20 gallon would be too small.
either way you'll need to do a good 50 to 75% water change daily.