They provide about as much cleaning help as adding another fish would. Just like any other animal, they turn food into waste (ammonia) and pee it back out. With giant clams, that ammonia gets cycled by the zooxanthellae so there can actually be a net reduction in N. Without the zooxanthellae to take it up, these guys only increase nitrate in the water. Not to mention since they need phyto, just feeding them adds lots of nutrients to the water.
