I assume you're talking freshwater clams...unless you can get them at some kind of petstore I'd be concerned about the fact that they sometimes inhale toxins and stuff. I'd want to know a lot more about keeping them domestically first. Do you have some already?
No I don't. Drs Foster and Smith sell them, so they're not going to be directly out of the Mississippi (where they would, indeed, be really polluted). Pretty much their only requirement is some food in the water column, just like with some of the marine clams. Like I said, the only thing I don't really know about is whether or not the betta, which is known for eating some other inverts, will try to eat the clam.
My female Betta killed a mystery snail within minutes of putting it in the tank. Bettas can be very curious and they tend to bite everything new they see to figure out what it is. There is a chance the Betta will try to "taste" the clam. Also, if the clam slams its shell closed to protect itself the Bettas tail could get caught. This would prevent him from reaching the top for air and cause him to drown. I don't think I'd put them together. Bettas and clams do not live together in the wild. I really think Bettas are best all alone.
Well...debatable Phloxface. It's worth researching certainly. My former betta Rusty was curious about the snail but never really bothered it; my current one Ribbon more or less ignores it. So it depends.
I'd be worried about the Betta's tail getting caught by the clam by accident. Bettas are curious and their long flowing tails get in the way. It's just not worth the risk. I just don't really see the appeal of clams I guess. They just lay there most of the time. I'd get the Betta and forget about the clams.
I have updated info regarding this topic. I got a couple of clams from Drs Foster and Smith. I was supposed to get three, but one had been an empty shell that broke open in transit. Because I only have two now, both are in the same tank in an attempt to breed them. (I doubt this will be successful, but you never know) This will not be the betta tank.
However, I did discover that both my worries and phloxface's worries were based on a false assumption. We were operating on the belief that freshwater clams worked like marine clams, sitting there with their mantles wide open. In truth, they only extend a little siphon tube from the tiniest crack in their shell. The chance of them getting injured by a betta and the chance of a betta getting caught by a clam are so close to nil that there might as well be no difference.
There is a point to that. I wish Liveaquaria had pointed that out.
However, in researching, they are only really harmful in large numbers or to stressed fish. Otherwise, it sounds like they just sit there while they grow a bit, then fall off when they can fend for themselves.
Still, I'm going to have to probably separate my clams once they have kids. Maybe start an invert only tank. (Been thinking about that, anyway.
Oh wait, clam larvae aren't dangerous parasites! Yes, they live inside the fish, but they do not harm the fish in a life-threatening way! If the larvae are old enough, they would get out of the host fish, drop into the substrate and starts growing as common clams.
I also found out the way they breed: The male would release sperm into the water, then the female would use a siphon to take in the sperm. Interesting, eh?
There is a possibility that if the larvae are concentrated in a small area (like a fish tank) that so many can attach to the gills that it hinders the fish's breathing, but you are right that in nature this is normally not a problem.