Are they translucent and without any particular color? Probably some kind of isopod or other *pods. Nothing to worry about, and a good sign that your tank is maturing.
They are called copepods or amphipods..they are a definite plus in a tank..they are a natural food source for many fish and inverts you will add to you tank in time...they, the copepods, will also help break down detritus and uneaten food.