Small aquarium of arguable definition. Freshwater nano-aquaria are generally accepted to hold between 5-10 US gallons (4.16-8.32 UK gallons; 18.93-37.85 litres) whereas in the marine hobby any aquarium with a volume of 30 US gallons (24.98 UK gallons; 113.56 litres) is generally considered to be a nano-system. These definitions vary, however. Also known as a nano tank.