"Last fish" ideas?

[GreenSpottedPuffer]

I am looking for either one more small fish for my tank or maybe a small pair of fishes. 20x20x20, 34G.

Have:

Pearly Jawfish
2 x Purple Firefish
Tailspot Blenny

As of now, everyone gets along extremely well and I have never seen the slightest aggression. I really want to keep it that way.

I had been thinking of a pygmy angel but then that probably means aggression especially since the tank is small.

Any ideas? Needs to be an active fish that will swim in the open a lot. My fish all either perch or just hover now. Oh and either red or yellow is possible but not necessary .