Okay, I am a fish keeper.... a 20L generally means 20 gallon tank, 30"X12"x12"..... I have misread... 20 liter? Oops... too funny...
I am glad that you caught my error. But I do want to add that there are several species of dwarf corys that only grow to less than an inch, which would be fine in a 5g tank. As well as several newly discovered dwarf rasabora or even badis badis, mid tank swimmers, that would also be very happy in a small groud in a 5g tank. Because of the passive nature of ADFs, a small group of small fish can be added. I personally perfer keeping corys with ADFs (as well as a couple of snails) to help scavange with the ADFs miss. And some keep a single male betta with their ADFs with success.
Thank you again for realizing that I misread the "20L" description.