I use black diamond fine in most of my tanks, but have never used the ultra fine. In my experience it's relatively quick to rinse out and has a straight black color. I think it'll look great against the fire eels.
I don't think it turned out too bad. This is 150# of sand in my 220. The sand is more of a dark brown instead of a black, but for $7.99/50lb bag, that's fine with me.