Jan 9, 2014 #1 Nathenrauch Aquarium Advice Freak Joined Mar 11, 2013 Messages 250 What do bio balls do in filters?
Jan 9, 2014 #2 NYgiantsRP Aquarium Advice Addict Joined Jun 6, 2011 Messages 1,872 Location Upstate NY They hold benificial bacteria.