I have the 36-48 inch orbit over my 40b and its been a few months and corals seem to like it. I have the higher light demanding corals higher up in the tank, such as a sps
frag. I';ve seen some growth on several of my corals but did lose a brain coral recently....I am still debating whether it is suffucient light though lol.
On a 20L it should do the trick as the tank isnt very deep like the 40b isnt very deep either. That and getting the largest fixture that will fir seem to be good ideas when looking at this light. So if you have a 24 inch tank get the 24-36 inch fixture, not the 18-24 inch one.
It is a very nice light though, i love it and the colors I get from it.