James, i've seen quite a lot of people with, and have some myself, Black Phantom Tetras. I've had most of mine a year now and they're no bigger than 3.5cm but nicely deep bodied. Very interesting displays they put on and do shoal nicely with 5/6 fish. Not expensive, very hardy and entertaining when they flutter against each other. I'd say get one male to 3 females. The females have a smaller dorsal and red on their fins. Girls only will display (i had 5 females to start with), but the males swim circles around the females when in together. Really like mine and change from the neons etc.