Clams rely strongly on light but are also filter feeders. Different clams have different lighting needs. Crocea and Maxima clams require the most lighting and without this light they will slowly lose their color and perhaps die. I don't feed my clams anything directly but I know they benefit from scraping algae off the glass/sand storms etc. They are filter feeders and do help keep nutrients low in the tank, but they need light first and foremost to live.
HTH
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