What should I improve in my tank?

I would definitely stick to what you've got. In my mind the real problem isn't that there are too few fish in the middle/top, but rather that there are too many at the bottom. When you get a bigger tank, you may begin thinking about something like a school of barbs (I've always liked the black ruby barb, but they don't look very colorful at the pet store. They only show their best when they've settled in your tank at home).