Ive found some hard, white organisms growing on the glass/rocks/sump in my tank — what are they?
Well, this isn’t easy to answer, because it could be a variety of things. In general, there are a few likely candidates that are common to new tanks, though. First, if it’s a tightly coiled spiral that is no more than a few mm in diameter, it’s most likely to be the tube of a Spirorbid polychaete. If it’s more straight or wavy than tightly coiled, it could be a Serpulid (I cover both of these with pictures in http://www.aquarium.net/1197/1197_1.shtml). Both are harmless filter feeders, which tend to go through periodic population explosions, especially in new tanks (they usually come under control or even completely disappear as the tank ages). Another other common possibility is that the critter looks more rice-grain-like, possibly with a tuft of “hairs” sticking out like a tiny funnel at one end. This is most likely a calcareous sponge rather than a worm, and you can find out more about sponges at http://www.aquarium.net/1196/1196_7.shtml. The final possibility is that the critter is
