Would the 30" Stingray (16w claimed) over a 20 long be considered low light? Not able to find PAR info yer, maybe my Google Fu is weak. I plan to do no co2, normal substrate, and low light plants.
Given what is shown on the finnex site, this would probably be MORE than enough for java fern, java moss, anubia, crypts, etc.. 20Ls are ideal given the nice footprint and low height.
FWIW, I grow all the above (but crypts) in my 29g with nothing more than a 24", 17watt, stock fluorescent hood with a quality Zoo med plant lamp in it.
But this video shows 30 par at about 17" under real life conditions (which tend to be lower than advertise normally)
So you would have plenty of light or maybe even too much light but certainly enough. It is easy to reduce light but you can't increase it so a stingray would be a good choice on a 20L.