What plants can I place

[Fireeel]

What plants can I use to fill the gaps in the background?

 

[Back in the fold]

That’s a nice aquarium as is .

 

[Fireeel]

That’s a nice aquarium as is .

Just need some fullness at the bottom.
Carpet plant needs to grow out.
Wood as a central piece replacing the rocks.

 

[MattW]

Since your stems are taking up the majority of the light, I'd go for low-light plants. Anubias or maybe some Buce. There are many varieties and sizes to choose from. You could possibly get away with a Java fern as well. But that would do better with higher lighting.