small plants for short areas

Hello All,
I've been reading this forum for quite a while and have been planning my own system to house some dart frogs. i have already started building the system and backgrounds and fitration but what i need is to know what kind of plants i can use to work with my particular setup.

I have a tank that is 48 inches wide 12 inches deep and about 21 inches tall. I think i came up with a way to make everything i need work, except the plants.

Since im limited to height i was wondering what species of plants i can use that might not grow tall but wide. Wide is ok but im limited to height obviously. This is why im posting. I'm looking for short, attractive plants to go with a tropical enviornment. Also what would a good ground moss be for this type of setup.

there will be a waterfall feature, and a fogger. all that part has been figured out i think, I just really need to figure out how to get some lush green foliage in here.

Thank you All,
Carlos