"Whilst there is no consensus for what is actually accepted as a nano aquarium, generally it is any tank that is less than 10 gallons."
"These Nano Aquariums is a fish-invertebrate community aquarium that is less than 20 gallons in capacity and can be the center of attention in any room."
"Nano is greek for dwarf and refers to small, compact aquariums not exceeding 30 gallons. Most common are 10 gallons or less."
So basically "nano" tank is a pretty meaningless term :wink2:
Some European countries have laws prescribing minimum aquarium size (about 54-60 litres I believe) which rules out anything below about 15 US gallons if you happen to live there!
For me, my "BIG" planted tank is about 45 litres (12'ish US gallons) and my "nano" shrimp tanks are about 11 litres (3 US gallon). So the concept of a 10, 20 or 30 gallon tank being called "nano" seems odd to me, but if you're used to having an indoor swimming pool, then I guess these would seem quite small in comparison!
I think most people call anything below about 10 gallon a "nano" tank though, but also depends upon what livestock and planting it contains (miniature plants and fish are more in-keeping with the "nano" theme).