What is the antonym for aquarium?

There isn't a single perfect antonym for "aquarium" because it depends on the context. Here are a few possibilities:

Focusing on the physical structure:

* Terrarium: Used for land-based animals and plants.

* Aviary: For birds.

* Vivarium: A general term for any enclosed space housing living organisms.

Focusing on the purpose/absence of a tank:

* Open water: Describes a natural environment where fish and other aquatic life exist freely.

* Wild: The opposite of a controlled, captive environment.

Focusing on the lack of aquatic life:

* Desert: A dry, barren environment with little to no water.

* Forest: Land-based environment with trees and other plants.

The best antonym will depend on the specific situation you're trying to describe.