I can be more specific on my clams to see if anyone thinks they will really have any noticable impact on nitrates. I just ordered 1 more Crocea (4") and 1 squamosa (2 1/2"). So I'll have 3 medium sized clams in a 230 gallon system with a medium load of fish. Right now I get up to .5 on nitrates in about 2 weeks, and a 20% change brings it back down to close to zero. I should be able to answer ther question myself after a month. I'll always do two week water changes as I have a simple set up for it, but I'd like to see nitrates stay below .5.