[Delphinus]

I remember this idea of using clams for nutrient reduction going back to the 90's and even sort of had high hopes myself at one point for thinking I would have a low nitrate tank because I had a dozen or so tridacnids. The reality is however, that despite that they feed on things like nitrates, so do corals feed on nitrates and phosphates and just having a tank full of corals isn't a guarantee to zero-nitrate readings. You'd probably need a vat of 200 clams sitting in a greenhouse to be able to measure an appreciable difference in nitrates in your average household reef tank. Personally, my advice is buy a clam because they're pretty and you like looking at them, and carry no expectation beyond being a pretty thing to look at.