There is nothing you can do for that clam. I would not start adding things to fix a problem that is not there. A clam is going to be tough in that small of an aquarium. If you want to try again I'd suggest a derasa or squamosa but they will out grow that system very quickly. A clam just might not be on the list of livestock I'm afraid. For the future I'd read up on some articles for tridacna clams, James Fatheree has some great write ups. Clams need good lighting and stable parameters. I mentioned new shell growth earlier, it's a positive way to know the clam is healthy and growing in which its using its carbon/energy to not only provide its daily needs but putting on shell growth. Clams do not do well in a tank stripped of nutrients so I would not try to get phosphate or nitrate to "0" by GFO or the like.