Clams do take in nitrate but think that it is not really significant in our tanks...
For anemone,
The down side for carpet is that they are really sticky and get really big... It is difficult to find small ones but still obtainable... It is quite a dangerous anemone I would say... Most fishes will steer clear of it... They dun really move a lot once they settle down, it depend on which species you are having.. From what I experience... Haddoni like to stay on the sand and Gigantea more to the cervice of the rocks...
BTA, a very unpredictable anemone, the one i kept alive was a RBTA which goes into hiding after 6months... Once settled it is very unlikely to move but a minor change in light and flow, it will move immediately... It is more likely to settle on rock cervice too...