Black Phantom Tetras in a moderately sized school look awesome in a tank around 18-20 gallons. You could get the little brother, Black skirt tetras. They look about the same, sans long dorsal fin. You could get, in the words of my friend lilfishie, a largeish group of neons, but in my experience, they tend to die a lot quicker than their exact look-a-like, the cardinal tetra. Or their not-so-distant cousin, the glowlight tetra. looks like neons, only orange. You could get a group of Serpaes, that is if you're prepared to stock semi-aggressive fish. There are also red eye tetras, all the fun of piranhas, just without the teeth. Then there's as my friend said before, lemon tetras. There's also Head 'n Tail Light Tetras, or Beacon tetras. Then, if you want to, you can get longfin black skirts or albino buenos ares tetras. They don't occur naturally, but they behave exactly the same, just look different. I have just recently had a BAD experience with Columbian redfins, but they're gone now. If you can find some peaceful ones, they are truly beautiful fish. In reduced light, their scales shine an iridescent blue that will catch even the blindest of eyes. They're kinda semi-aggressive but they'll get over it after they've established the pecking order. There's also the Bleeding Heart Tetra. Kinda big but just small enough. Did I help any?