James Fatherhee, one of the big clam guys, recommends Ca 420ppm, and dKH of 7. I don't remember directly or not if he makes mention of magnesium. You can check out his book if you are really interested (
http://www.liquid-medium.com/giantclams.htm).
Clams can be pretty basic. They are photosynthetic, and thus require a decent quality of light. Small clams though, probably under 2" require additional feeding help, as their matle isn't large enough to help generate the food they require. I've had small clams 1" or less that I didn't necessarily feed tough, so its not a hard and fast guideline. Some clams (squamosa, I think) can be kept under VHOs, however, I've never done it myself. If you want a nice "simple" clam, try a squamosa, or a maxima. Croceas are beautiful too, however they are little piggies when it comes to light. Gigas are stunning, and get quite large. Derasas are unique with their scutless shells. There are also other clams we don't frequently see in the hobby.
What kind of lighting to do you have now?