Floating Moss Balls are a feat of engineering and patience to create: it's basically a hollow plastic sphere (think Ping Pong ball!) that moss has been trained to grow on like ivy. The moss is carefully and invisibly tied on with fine nylon thread and anchored to a suction cup or weight so the ball bobs at whatever height you set the wire to. They look like underwater mines or balloons and are most effective when you have a group of 5 or 6 at varying heights.
They are very easy to care for - they only need low-medium light and a gentle filter system to prevent debris settling on them. Remember to turn them over every few days to allow each side access to light or you'll find the underside will brown-off in time. If they start to look more fuzzy and over-grown than you'd like, just trim them with scissors - it's like aquatic topiary!