IMO, lighting is a major issure in keeping clams...but not the ONLY issue! Water parameters, nutrient levels and water flow which MUST be moderate at the least, with turbulent flows periodically and the avoidance of laminar(straight line) direct stream water flow!
Now, having said this, Crocea and Maxima's are the most difficult of the Tridacna's to keep. They should be positioned higher up in your tank and situated among the live rock and not in the benthic area of your tank. Unlike their counterparts, Hippopus, Derasa, Gigas and Squamosa clams which should be placed in the substrate area of your tank.
Remember also, that theire are many variables that will affect the amount of photosynthetically available radiation(PAR) that is absorbed by clams, and corals as well as other symbiotic invertebrates. DOM, turbidity, glass tops, suspended matter and particles as well as plain salt water, all play a part in the refracting of light back into the environment, which in essence decreases the amount of needed light for zooxanthellae to process this light as energy and as an end result produces ammonium which is a needed food for corals and clams!
Let us know how things go with the clam aquasition!!! _________ 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  | 
