The 20g will grow the crypts and other things fine with 15W of T8, so basically stock lighting is totally fine for most low-light plants (java fern, mosses, crypts, swords, etc). No CO2, no fancy substrate. Check out this thread...
http
/www.fishforums.net/index.php?/topic/295754-endor-lljs-36g-low-light/page__p__2443246__hl__endor__fromsearch__1#entry2443246
Was a 36g tank with .77WPG of T5 lighting. As you can clearly see, plants grew, and grew well. I had anubias, crypts, E tenellus (a 21" deep tank too, pelia, Aponogeton crispus). Inert substrate, only dosed TPN+ when I remembered. About 10ml.
But then, it could well just be me, because it's me, and I'm fabulous. We've established my fabulousness from the beginning.
L