I have used 0,6-1mm quartz from the home improvement shop. Washing losses were accepteble.I am interested that some have had good luck using sand as a cap. Is the sand the same size that is used in tots' sand boxes, 1 mm or less? For how long was the aquarium in operation? I am wondering how the soil was replenished, since the sand cap would seem to prevent fish droppings, etc., from getting to it.