If your pH is really 7.4, I don't know how ANYTHING is surviving. That's big deal - that's freshwater pH. The difference from pH 7 to pH 8 is not 1, it's 100. You need to get a lot more aeration in there - that's often the cause of low pH. More pumps and water movement, a protein skimmer, hell, air stones are better than nothing. Also, I assume you meant calcium at 400, not carbon. And there should be NO phosphates. Are you using RO/DI water? Phosphates interfere with calcium uptake, and clams need boatloads of calcium - 400 is good, but at that pH and KH, combined with phosphates, calcium may not be available for the animals to use. Your KH should be at least 8. You've got serious problems - you should not attempt to keep a clam unless you have better water parameters.