The 20L and the 29 have the same footprint. The 29 is just taller.The one currently sold is great for a 20g long. I had a great amount of growth of plants in my 20g long with this lighting. I read a review somewhere that it's even good for a 29g. I have a HO version from AquaticLife on a 29g, and that thing is giving me a lot of algae, lol!