I think NAS's clam-to-non-clam ratio and fairly low overall water volume comparatively probably contribute to that.
I've read the same stuff about people adding nitrate for the sake of their clams and all I can say is I have never had the need to do so. I have 15 or 16 in my 110g, I've totally lost count, ranging in size from 2" to 15" and I have never had low nitrates, never. Fish load is fairly nominal, 5 fish in a 110, but even before when I only had 2 fish in there. Over ten years in the hobby I've only once detected a minor drop in measurable nitrates after adding clams, but it was a short lived dip because the levels returned to normal for the tank shortly after.
I'm not saying don't have clams, and I'm not saying clams don't reduce nitrate, I'm just saying it's hard to plan for a clam population for the actual sake of nitrate reduction in a tank. If you find that your nitrates are always zero then you can add some back to maintain say 5.0 and likely the clams will respond positively but I wouldn't be adding nitrate unless I found that there was literally an overconsumption of nitrate .. I myself can't say I've ever encountered that myself but like anything, "YMMV".
