The chances of survival with a clam of that size is very slim!!! I would invest my money in a clam in the 4" range. A clam at that size requires strong light which you have for its food and survival. Their iridophores and zooxanthellae have grown and will provide nutrients needed for survival.
Clams as small as you are debating the purchase of require food(NOT LIGHT)! And more often than not the food we give them is too big for them to take in and digest, thus the motion of them almost going through a coughing motion somewhat similar to the reflex of a change in light over the clam causing it to close briefly. Nano plankton and yeast, yes, yeast is what they prefer! Yeast used to make bread, found at local supermarkets everywhere.
IMO though, I would shy away from baby clams and focus on juvenille to adult clams for long term success as I have had both and lost the baby ones while I have kept a Corcea and Squamosa for a long time now and they even made it through a house and tank move!!!!!!!
For an additional $20.00 roughly, you can get an adult clam(prefferably an ORA clam) with a greater proven track record!!!!!!!
One last thing to take into consideration, you will have better odds with a Squamosa or Durasa tridacna than any other clam!!! They are hardier and more likely to survive many years of pleasure for you in your tank!!! _________ 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  | 
here's the thing i just recenly moved into the new tank say about 3weeks ago, im used every last drop of water form the old tank about 20g worth of water,then i added those live water that u can buy in a 4.4g jug and the rest is fresh mixed water. the way i look at it i practically did a 30% water change. all my other corals are doing good all opened up to how they were in the old tank so...... am i ready ???
Did you transfer any live rock or sand from the old system into the new? If you did, you can bet that there will be some bacteria die off as a result and your tank will go through a mini-cycle, assuming it hasn't done so already seeing as you moved the tank 3 weeks ago.
