I think its really a mistake to say clam remove the no3.... yes they do but only in very huge numbers... that one clam of yours will not do anything much... BTW whats your lighting like... you say your other corals is fine?? what kind are they.... IMO clams are not really suited from instable and young tank, they aren't that easy as they seem... IMo more suited for a matured tank with proper equipments.... if your clam is used to suck nitrate, i think you better sell it away....