Moss balls a great addition to the tank. Many fish love to nibble at them for a little snack, they help absorb excess nutriments in the water that algae love, so helps with natural algae control, and I think they look really cool in a tank.
i love my moss balls! i have three in a 20 long right now.
aviva made a very good point about them, sucking up the excess nutrients to help keep algae at bay. funny enough, marimo moss balls are actually algae!
Yup its a type of algae that gets rolled around on the floor of the lakes they are in causing the "ball" shape. "a species of filamentous green algae (Chlorophyta) found in a number of lakes in the northern hemisphere" -Wikipedia (just a short explanation)