| Could be a ton of things. If you would really like to see a cool sight, find a source of light that is red (flashlight or anything) and look at the substrate at night with that and you wont believe the activity you will see all over the place! I do this once in a while and as my tank matures, it only gets better! The red light cannot be seen by the fish or critters aparently. (Saw that on a recent episode of 'Blue Planet" where they used red light at very deep water so as to not scare away the usually lightless fish and inverts.) | 
