Join Date: Feb 2013
Location: San Antonio, TX, USA
I was wondering what sort of lights do you have? I think you may get stunted growth with regular lights in that tank. It looks tall so I'd add more lights - probably closer to 4 watts per gallon at the 6500K range. This is a guesstimate, but your taller tank will prevent light from reaching the lower depths. You can read more about PAR and other lighting criteria by googling it.
Now, the plant you have now, the moss, will probably grow stunted with your current light. Another thing about mosses is that they prefer colder water. If you don't have fish/invertebrates, I'd turn off the heater. You may get more growth with that, but it won't be great unless you upgrade your lights.
As far as the substrate, I think that gravel is a bit too big for planting. Plants need smaller gravel and/or sand to expand their roots easily. It will still work, but may take more time.
Another thing to consider adding is CO2. I am a low-tech type of guy so I'd either do a DIY CO2 system (you can google it, you need 1 bottle, plenty of aquarium hosing, sugar, yeast, and water). You can't go wrong adding extra glutaraldehyde (also known at Seachem Excel or API CO2 Booster).