I will assume all water params. are satisfactory (also test for sg
, pH, Alk
, and Ca
) so have you noticed any fish aggression (what fish are in the tank?) towards it or other invertebrates picking at the mantle? If the clam is now located on the substrate (doesn't look like it from older pic) pick it up and check the entire shell for pyramidellid snails, flatworms, etc. Sometimes clams will close for a couple days only to reopen as if nothing is wrong, but better safe than sorry.