A 20long, is in fact longer and will need more power as far as flow. i personally would use two 425's if you want to keep coral. If adjusted correctly they will not blow up sand or anything else and will give your flowy corals like lps
I ran a 750gph powerhead in my 29g biocube and managed to make it work, lol, but most of the time used a 425 and it was just enough and a biocube is alot less long than that 20L tank.
I would aim for 20lbs at least of rock, you want a strong biofilter and enough room for corals. With it being extra long you can do some really sweet rockscapes in there.