I disagree.
I have a 20g long which is about 14 inches deep with sunstrate + water and I grow low light plants very easily.
I have a Beamswork 78 LED (0.6 watts each) and the fixture is 16 inches total from the substrate.
I grow Elodea/Egeria Densa, Java Fern, Water Sprite, Twisted Vallis and...