Most copapods live off of plankton and some agal. I do not think that a non tropical copapod would be a good addition to a reef tank when considering thier specicfic live history stratagies, niche breadth, and different physiological biolimitations.
Feather dusters are invertabrate filter feeding worms that will open up as a feather and filter the water for plankton. Many inverts, micro-crustations, and decapods just to name a few will "hitchhike" on your live rock along with nitrobacter.
A diatom Filter is one with very small filter pores that trap just about everything, this is why it is used for "seasonal cleaning" and is not ment to be ran alll the time. It has the ability to capture diatoms which are the brown hairy slime you see in some tanks.