Tank mates for Zebra Danios?

I'm going to get some tank mates for my zebra danios soon, but I'm unsure what to get. I have a 20 gallon freshwater aquarium with 5 zebra danios already. I want some fish that stay about in the middle of the tank, because my zebra danios seem to like the upper and the middle regions of the tank. I heard that some fish can't handle how active the Danios are and get stressed out, so I'm a little worried about that. Please don't suggest Cory Cats, I love them, but not for this tank in particular. Thanks in advance! All suggestions are loved and welcomed!