I don't know what the height is, I guess 30cm? You might already have some, but the only plural thing I personally would put in a tank that small is shrimp. The smaller the tank, the less forgiving with parameters and in a planted tank shrimp almost clean up after themselves! I love my shrimp colony's though, the red cherry's especially spend a fair amount of their time out on the plants feeding rather than hidden away, they need minimal swimming space, leaving plenty of space for plants!
Although plenty of people put a single betta in something smaller.
I have fry in a lunchbox, but my feeling is long term most other fish would want more space or need tank mates.
I'm also sure someone else will have better suggestion though, just wanted to put my vote in for shrimp because they were what got me into keeping fish.