If you keep parameters suited for sps, then you can keep a clam. Clams that are aquacultured that are 2" to 3" are completely fine and a good choice. Large wild caught clams over 4" to 5" can be more difficult to keep, but not impossible. Derasa, Hippopus and squamosa are pretty good starters. I think derasa and squamies are easier to come by though. All three get pretty big, so plan on making room. Maxima and crocea need more care (high light is required), but stay smaller. T. noae (aka teardrop) are very cool, but more expensive than others for a basic looking clam, but they're cool. I put their care level up with crocea.
As far as feeding and flow goes, it's true that you don't want high direct flow, primarily to avoid pinning their mantle back, but it's also kind of a myth. Clams will orient themselves in high flow areas that suit them, so don't be overly concerned with flow, they can handle high flow just fine. The only time high flow becomes a problem is when they can't extend their mantles. I've snorkeled in Tahiti and there's no way that we can replicate the amount of flow in our tanks that these clams were experiencing on the reefs, so I wouldn't worry about flow. The other myth that needs to go away is feeding clams. It's true that clams filter feed, but this is not their main source of food, zooxanthellae in their mantles is their food. So strong lighting is key here. Phyto is not required to keep clams and completely unnecessary. The only time that I've seen phyto help is when clams are kept in lower light tanks and supplementing with phyto made up that difference, but even then, phyto can only go so far before the clam(s) suffer and eventually die.
Personally, I would recommend that you slowly lower your alk to more natural levels, but that's just me. We used to keep alk in the 11's and 12's during the 90's, but that's what everyone did back then. I'm old school, so I would recommend a few books (that's right kids, books) to read about clams. Daniel Knop: Giant clams and James Fatherree: Giant clams in the sea and the aquarium. These are great resources (as are a lot of books about the hobby). There are a few places that have clams online that I would recommend, ORA, Pacific East Aquaculture and Clamsmania (never ordered from him, but hear good things). I'm sure there are a number of LFS that have clams, which I always prefer seeing clams in person. All the clams I listed can go in very strong lighting, but only the larger species can thrive in lower light (200+ par). Derasa and squamosa are probably good choices as starters, but as I mentioned, they get big and fast. The key with keeping clams is always checking that there's new growth on the shells, if you don't see that nice, crisp white shell growth, then something is wrong and most likely the clam is starving and will eventually die. Always check for pyramid snails with new clams (or any snails you buy after you get a clam). Brush and clean off their shells and don't be afraid of knocking off a few scutes if you suspect pyramids. Sorry for the long post, but I absolutely love clams (and reefs). Take your time and find a nice clam, but do your homework.
Cheers!
