I removed a large bristle worm from my tank about a year ago and kept it in a bottle for a few days, did not have the heart to kill it. Returned it to the tank and now it is over a foot long by far, comes out everyday to feed right after I feed the fish. Great CUC, and has never touched a clam or any other tank inhabitant. Some people use them in their clownfish rearing tanks to clean up the dead fry and any remaining food that could pollute a small tank.