Could get a group of pearl danios. Any danio would work just the same, really, but IMO, pearls put on a better show than others. Once they're settled in, they're a blaze of red and blue, not the boring pink they usually look like at the LFS. Mine are a bit dumb, though, and will try to shoal with any blueish fish in the tank, usually getting themselves nipped for the trouble.
Not all killifish are overly aggressive, but since you have high filtration, I'm guessing there's a great deal of surface movement as well? My 30 gallon tank had very strong surface movement, and he didn't like it, and tended to lurk in the bottom of the tank. My 55 is still filtered almost 10x volume, but has much lower surface movement, and he doesn't mind as much.