I have these also. I have been told they are in the catagory of Copapods (but are hard like a barnicle) They are benifical to your tank and are food for other animals. You should scrape them off the front and sides, or they will obstruct your view, but let them grow on the back of the tank. If you look really really close (use a magnifine glass you can see little arms growing from the sides)