Yeh the moss balls are fantastic for collecting dirt though it's not so fantastic on the eye. I imagine steverade is right, I have experienced just that with my fish and moss balls (algae balls). If however they're getting torn to shreds though I would say, not long.
Thankyou for the replies, my yellow lab is the main fish that has been ripping parts off. Are there any other plants that have the same role (collecting dirt etc) as the moss balls do?