not only calcium/alk, but your also have to consider the parameter swings in smaller tanks- ie- salinity, temperature, etc. It is markedly more difficult to keep a clam in a small tank than a large due to these reasons.
Nudilove- gigas does not need the least amount of light. This is a dangerous way to think, also. clams need a lot of light, and figuring out a way to get the bare minimum will likely end in failure. If you want to keep any clam, get as much light as you can, and then consider whether a certain species would be appriopriate.