never had one, tanks were always fine. When I set up my new tanks (720gal and 200gal) put one on the 720 and NOT on the 200 gal. Did nothing. Both tanks are fine, in fact the 200 gal is clearer (for other reasons).
IMO, waste of money, overhyped. I took mine off the 720 just yesterday. No need.
Algae (greenwater) not a problem and as far as parasites, etc, I don't add any new fish to my tank , so no big deal.