occur more easily, and so nano reef aquariums require extra attention to water quality compared to aquariums of larger water volumes. Many experienced reef aquarists recommend testing the water twice weekly, with water changes at least every week. In particular, ammonia, nitrite, nitrate, pH, salinity, alkalinity, calcium and phosphate levels should be monitored closely. When it comes to nano reefs, even minute changes in water conditions such as mild temperature fluctuations can be problematic, whereas the greater water volume of larger aquariums provides a more stable and flexible environment. Nano reefs also require extra care in the selection of occupants. There are two major factors to be considered: biological load, i.e., the ability of the tank to process the wastes produced by the occupants, and species compatibility. These issues, though present in larger tanks, are magnified in the nano tank. Species considered reef safe and able to coexist in larger tanks may not do well in a nano tank because of their close physical proximity. For this reason, smaller species of fish, such as gobies and clownfish, are popular choices because of their relatively small size and ability to coexist peacefully with other tank inhabitants. Filtration in nano reefs Many nano reef aquarists prefer their displays to be as natural-looking as possible, and therefore choose to use as few mechanical filtration methods as possible. A primary filtration method in nano reefs is live rock and live sand, which are pieces of rock and sand that have broken from the coral reef and are populated with beneficial bacteria and other organisms that aid in breaking down organic wastes produced by larger organisms in the nano reef. Other nano reef aquarists use devices such as protein skimmers to remove excess waste from the aquarium, before it has a chance to be broken down to nitrate. Removing the excess wastes mechanically can reduce the frequency of water changes needed to keep nitrate levels low. Delaying the action of the mechanical filters, such as by means of a day-night timer, can allow invertebrates to filter-feed naturally. A refugium may also be used to export nutrients, when packed with macroalgae such as Chaetomorpha, and live rock. Deep sand bed filters are another filtration method. Recently there have been several "natural" methods of processing waste in the aquarium and specifically small environments as nano-reefs. The research on the encouragement of the development of different types of