I've kept two species of FW clam so far, with less than outstanding results. The one I have not tried to keep is the Golden Asian clam.. a local store has them, the owner is a great guy who knows a lot and says one clam, one inch diameter, is all he can keep in a ten G tank and have it survive. He says they are actually quite heavy feeders, for a filter feeding critter.
The clams I kept were the rounded black/white ones, and one called a sharktooth clam. The sharktooths lived only weeks, I suspect they were already starved beyond saving when I got them, but very interesting appearance, with a very large shark tooth shape protuberance on the shells. They did not dig out of sight, but that may have been because the substrate was not deep enough to allow this.. or because they were weak. I am not sure. I have video somewhere of them digging in though.
The round ones, well, one lived nearly 18 months. Two lived nearly a year, and that's better than most reports I've heard on them. They did gain some weight, which is the only real way to judge how they are doing unless they are putting on visible growth, and you have to dig them up to see how they are doing.
All the clams I've seen appear to prefer to bury themselves if they can. You might see the shoulder. Usually it's just the feeding/breathing tubes. If they die, they open up, and if you have snails or shrimp they will be cleaned out long before they mess up the water parameters. I could not say if they are edible nor how long they'd take to get big enough to eat.
I did find they needed supplemental feeding. I had my 3 rounded ones in a 30G tank. I fed them Golden Pearls in the 5-50 micron size, cultured green water [ single cell algae], and I weighed them monthly to see if they were doing ok. In light of what my local store owner told me, I think now I underfed them by quite a bit.
I think they prefer cooler temps than even my cool water fish. I had Danios, kuhli loaches, cories and shrimp in with them. You won't see them if they're happy and healthy as a rule. They dig back down pretty quickly if you dig them up to weigh them. They don't move much once they dig down, not more than an inch or two from where they went out of sight, so finding them is not necessarily that hard to do.
I would not recommend them for any small tank, but if you have a big tank, and it's cool, and you are sure of loads of infusoria being available, you might try one to see how it goes. Big golden clams are not cheap, at least not here. Maybe where you are they're not costly. I'm debating trying one of the little ones my local store has.. but haven't yet got one.
Edit. I've seen a recipe somewhere for a clam diet.. I think it's on my old crashed drive though. Had beef heart in it, I recall.. and had to be finely pureed to feed. Poster of the recipe was successfully keeping clams, so she said.