I have a zillion of them, yes I counted, a zillion throughout the bottom of my 400g display. I have the wrasses, angels, and peppermint shrimp and nobody touches them. I love that I have a solid field of them. This is mother nature giving you biological filtration. Especially in a 15g tank, the more biological filtration you have the better. Mine stick to the bottom of the tank, and my tank is bare bottom.
Dave B