im sorry, but i dot completely understand what your saying. who is they and what kind of nano tank are you talking about (brand or just a small tank). live rock, or more often live rock rubble is put in the sump for aditional biological filtration. rubble is used because it provides more surface area. chaeto is used because it removes wastes from the water in order to grow. these wastes would usually feed algaes. then all you have to do is occasionally remove some chaeto and you wont have a problem