Can you have water in an aquarium?

I'm thinking of buying a freshwater aquarium, but I've seen videos on Youtube of aquariums and water are only fish. No ornaments, rocks, substrate. Nothing but water and ect filter, heater. It can be done? or must be somthing in there?

Some people use tanks without ground fish breeding tanks, hospital tanks, tanks scientists screen. Emphasis is placed on certain types of fish, if they have nowhere to hide. The fish die or are more susceptible to disease if stressed. marine aquarium use crushed coral substrate, usually to stabilize a lot of things in the tank as the pH, calcium, some systems give a place to have a cycle of nitrate.

Aquarium Long Rock – for fish and reptiles