Depends on who you talk to but I think these guys are hard to keep in that they require regular effort on the part of the aquarist to ensure they are getting enough to eat... I'd try a plankton like this:
http://www.drsfostersmith.com/produc...m?pcatid=14719
You may need to turn off any powerheads for a ten minutes or so and use a feeding tube or similar device to target feed them plankton. You'll have to feed in an area a little bit away from them because if you try to put it right by them they may close up. Try to get the food to drift into them. They can also be at risk with fish nipping the mantle. Good luck with this clam, pics would be good too!
Mike
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