“Nano aquariums” are simply small aquariums. There are very roughly two different “nano aquariums” in the hobby. One is under five gallons in size and one is from five to fifteen gallons. They will be treated separately.

Under Five Gallon “Nano” Aquarium
One special situation is when someone has a small one half to five gallon aquarium and wants to add some colorful fish. They need something called “nano” fish which do not require schooling. Such fish include Endler livebearer, scarlet badis (Dario Dario), pygmy sunfish, dwarf chain loaches, peacock gudgeon, sparkling gourami, Tanganyikan shellies, and the betta. My personal favorites are the Tanganyikan shellies (Lamprologus ocellatus and their relatives). They are very small cichlids with great personalities.
https://aquariumscience.org/index.php/17-9-nano-aquarium/