Nicrew looks good, I like that this model seems to have more white lights than some of the others. better for fresh water in my opinion.I've heard of people having pretty good luck with the Nicrew lights. 20 gallon longs are only like 13 inches high and a quick google search shows that at 12 inches they have par rating of 60-100 at 12 inches depending on the model. Based on what plants you have you can choose accordingly. However, I would also consider the fluval 3.0 mentioned previously. You can dim it down for a low light setup, but it would give you options for plants that require strong light in the future.
I can see that being an issue on the 20L you will want to light up the whole tank. That's why I bent the metal braces for the off brand LED bar to raise about 3 inches. but in the 75 I can get away quite easily and the fish like having some darker corners up high for them to escape to from time to time. Having this "safe" and dark zones in the corners do two things, fish will head up there if they are not happy, and the corners are where my filters in and outputs are so they get a little hidden. That being said I wouldn't mind raising it an inch or two. But I don't really seem to have many down sides yet.I have a 20 long and have both the Finnex 24/7 and Fluval 3.0.
I like that the Finnex spans the entire tank giving better coverage, but being able to customize the photo period more accurately with the 3.0 was my deciding factor. Raising the 3.0 up about 5 inches off the rim helped the coverage issue.