^^^
mercenaria mercenaria require sand or mud, at least in the wild. The bivalves with spines are definitely a different specie.
Mercenaria mercenaria are not from tropical waters. You'll see them in the stores covered with ice. That ice isn't there to keep them from spoiling it's there to keep them from dieing.
It's like snails, you can go down to the local beaches and collect snails (legally or not) and they won't survive long in water near 80 all the time in our tanks. I would imagine it's the same with these clams. Any reduction in nitrate would soon be negated when the clam dies in the system and goes unnoticed.