+1 for MFD. I have tanks that far exceed the inch per gallon rule and they run great. I figure I've got about 75-100 inches of fish in a 20L. They're mostly guppies, so it's not bad. I wouldn't even dream of putting an 8" oscar in a 20L.
Lol, 75 guppies in a 20L? I can't imagine what feeding time likes like
--Adeeb