I have the 15 gallon column tank from Petsmart and it is also 24" tall. That will be a low level of light. Cooincidentally, I am growing anubias, java moss and fern, and crypts as well. Because I have some hair grass and some stems like mayaca as well, I bumped it up to two fixtures. I get pearling from everything down to about 8 inches or so from the bottom. hope that helps.