As your sand bed matures, you may notice tiny pockets of air, along with distinct layers at varying depths. The tiny air bubbles are most likely going to be nitrogen gas, which is a byproduct of bacterial respiration deep within the sand. The bacteria that convert nitrates to nitrogen gas live in the anaerobic portions of the sand bed (areas lacking oxygen), and the harmless nitrogen will likely bubble out of the water. Be careful when performing maintenance on your aquarium and do not disturb the sand bed. The anaerobic bacteria also release hydrogen sulfide, which could get back into the water and wreak havoc on your livestock.