Organisms that feed on smaller particles such as detritus and its associated components, as well as zooplankton, can either be fed or, in some cases, ignored entirely. It has been my experience that many forms of fanworm will exist quite well in aquariums, where the fish are fed regularly, without any additional feedings. If you find that the crowns of these fanworms decrease in size, then this may be a sign that they are not getting enough to eat and supplemental feeding would then be required. Liquid foods work well for the smaller species while live baby brine shrimp will be accepted by the larger species i.e. Sabellastarte magnifica.
From...
http://www2.hawaii.edu/~delbeek/reefaq5.html