It depends a little bit on whether you want a fish swimming around the open water (in which case I would 100% recommend Orchid Dottyback (or a dottyback of similar temperament)) or if one that swims around a lot but mostly in and around the rockwork would be fine (in which case I heartily recommend a Tailspot Blenny or a close relative of it). Both orchid dottybacks I have had were constantly zooming around the tank. And IRL, their purple scales seemed almost neon under my LEDs. So they would be my number one recommendation for you, with tailspot blenny as a close second.
Fang blennies (Meiacanthus sp.) are also active open water swimmers and most every species of them in the trade is available captive bred, which is a nice bonus.
Midas blennies are another option, but they get pretty big (5" long) and personally I wasn't comfortable putting one in my Lagoon 25 for that reason. But idk, that's your judgement call.
The pygmy wrasses (six-lined, pink streaked, Tanaka, Possum, etc) are all very active swimmers that love to zoom around tanks. I've heard some of them can get really aggressive though, so just do your research on the species if you decide to go that route. As far as other wrasses go, I would never recommend a non-pygmy wrasse for a nano, although they do have the activity level you are looking for.
Firefish are a bad idea IMO because yes, they are always swimming, but they are swimming in place. They like to pick a spot in the tank, and just hover there for a while.