Spirorbid polychaetes are harmless tightly coiled white spirals that are no larger than a few mm in diameter. They appear on the glass, pumps, in sumps, etc. These common critters are most often seen in new aquariums, and may have periodic population growth depending on nutrient availability. Spirorbids usually come under control or disappear as the tank ages. They feed primarily on tiny particles in the range of bacterial floc and tiny suspended particles of organic detritus to the smaller end of cell sizes of phytoplankton.