The exact limit that distinguishes a nano reef from a regular reef is somewhat ill-defined (some claim that anything less than 180 litres/40 gallons would qualify), but 140 litres (30 gallons) seems to be the generally accepted limit.
The exact limit that distinguishes a nano reef from a regular reef is somewhat ill-defined. But 140 litres (30 Imperial gallons/37 US gallons) seems to be the generally accepted limit.