best clam supplement?

supplement as in food or do you mean additives...if you mean additives you try sponge power by Zeovit. (I have no experience with it but most zeovit additives seem to be very good.)
 
How big is it? Chances are that it does not need to be directly fed or have anything added for supplementation. Just keep your params right and let the light and fish do the rest for you. I have 3 in my tank and they do more than fine with just what I said above.
 
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