I've read (never owned a clam) that the biggest thing to avoid with Clams is agitation of your substrate, IE, flying sand and other junk. They breathe in the water, and if there's sand (or really anything else large and inedible in it) they get cut and clogged. It's like a human trying to breathe in a sand storm. Think about how little it actually takes to make us cough... now think about the clam's case as being similar, just with "thicker air." That said, pumps and hands both can agitate your substrate, so try to avoid agitation from both.
Also, since clams will attach themselves to a rock, and cannot be trivially removed, you should be careful with placement, especially considering that in the future, you may need to agitate the substrate, or otherwise get the water dirty. (My idea is a dedicated "platform" stone for the clam to sit on, but which can be removed. Others may advise why this is a good/bad idea. I lack the experience to know for sure.)
(No, I'm not considering a clam at all I swear... not doing any research on them and lighting requirements either... really... don't you believe me?
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