The size that a tank has to be to be classified nano is kind of up for debate all over the internet.
Some say 20 gal
or smaller others say 10 gal
or smaller and others say 29 gal
or smaller or 15 gal
The real limit of what is or is not a nano is less important than knowing that nano = small tank.
Freshwater folks might call a 20 gal
a large system but us salty folk usually think of a 20 gal
as a moderatly small system.