yes, xenia consume dissolved nitrates, nitrites, and ammonia, there is an article called "down the drain nutrient export in marine aquaria" xenia was shown to contain a minor amount of nitrates a moderate amount of nitrites and a moderate amount of ammonia. as we akll know ammonia leds inevitably to nitrates........
little necks (quahogs) are definatly not well suited for reef tanks. the salinity is higher than they like the temperature is higher than they like, and they require a large amount of planktonic foods. they are better for FOWLR tanks run at a lower salinity, with no skimmer than they are for heavily skimmed, high salinity reefs. if yuou dont have a skimmer on your tank, a quahog may help, long established tanks, they may make it, cooler water tanks
, tanks fed Lots of phyto they are good. most new reefs (or smaller reefs) just dotn have the planktobnic content these clams require, they do not photosynthesize, they do not absorb nitrate, they filter particles that may lead to nitrate, and move through the sand bed helping keep it aerobic, nothing more.