Both of those plants are houseplants. Put them in a pot by a window, they will do just fine.
If you have 15 watts over a 20gallon high tank, you are very limited in the type of plants you could grow. You can use mosses, true aquatic ferns (java fern and African fern for example), and any anubias species. None of those plants need any type of substrate, as they grow attached to decorations (like rocks or driftwood).
A plant you can grow IN the substrate in low light would be any of the Cryptocoryne species. They are not fussy on the type of gravel. You can also use sand with them.
Check out this link:
PlantGeek.net - Plant Guide
With it, you can search by lighting type (you have low light) and also see what to avoid when going plant shopping (they have a non-aquatic plant section).
Good luck!