I've kept clams, but the longest any of them lived was 18 months or so. I think most species in the trade need cooler temperatures than most of us provide. And they don't clean water, as the stores so often claim.
Only a very mature tank will have even remotely enough micro fauna in the water column to keep them alive. The only way to be sure they are eating and growing is to weigh them every month or two on a sensitive scale.
I fed mine cultured green water and other fine foods that I feed my filter feeding shrimp and some of them did ok for awhile, even gained some weight. But eventually they died, from unknown causes.
If they die, and you don't have shrimp in the tank, they can make a mess of the water, spike ammonia. But shrimp will soon clean their shells out, and they find them very fast, as the breathing tubes are above the substrate. Snails will finish the clean up.
The only species I've seen any accounts about, that lived any length of time, were the Asian Golden clams..and they are not often seen. They can get to six inches or more, I believe. But they still need to be well fed, and it has to be food that's very very fine... essentially the size of bacteria or copepods and that type of thing. Microworms, are, I think, taken by them but are not a whole diet, unfortunately.
Overall, I don't think they make good pets, though I still like the look of them. But you should also know if they are healthy you won't be seeing much of them, because they dig into the substrate. Usually all that shows is the breathing/feeding tubes.. or perhaps part of the shoulder of the shell. Some species dig in at an angle, so you see part of them, but the small white/brown/black ones that are rather roundish, dig right in and vanish.
