What size Crocea was it? They won't eat phytopalnkton(atleast not the kind sold in liquid form). The plankton is too big and they will basically spit it out like a cough or sneeze.
They feed on nano plankton, yeast, DOM and nitrates and as they get bigger and their zooxanthellae build up they will use light foor food as they will feed on the waste from the zooxanthellae during photosynthesis.
Crocea and Maxima's are the hardest of the clams to take care of and are the most light demanding Tridacna's!
Sorry to hear about your clam!! I have had a couple clams die as well, and as you you are like wow, what the heck happened...it was fine?
Water parameters are very important as are eliminating changes in salinity etc. to quickly.
I have had three tridacna's now for atleast 8 months now with no problems(knock on wood) Squamosa, Gold Teardrop Maxima, and a blue Crocea.
I would try again, if not a Crocea again, maybe a Squamosa or Derasa! _________ 125gal.w/Mag9.5 return(dual megaflow)>Mag7 pump Aqua Cev180skimmer.Wave2k Hamilton Reefstar(2)250watthqi(mh)pend.a Yellow, Naso Tang Red Lip Blenny Percula Clown Demoiselles Niger Trigger F. Wrasses Ceriantharia Orn.Shrimp and Stars Hermits Queen Conch asst. snails> Stars Zoos shrooms Montipora Brains Gorgonians Favia Turbinaria(large+small polyp) Acropora Xenia Tridacna (CroceaMaximaSquamosa) Leathers <35+75gal.reef tank as well> 
"IF THE PHONE DOESN'T RING...IT'S ME"  jb  |