Here's why:

* Solids have a definite shape and volume. Fish have a distinct shape and a fixed size.

* Liquids take the shape of their container and have a definite volume. While fish can move around in water, their bodies don't change shape to fill the container.

* Gases have no definite shape or volume and fill the container they are in. Fish don't behave like this.

Let me know if you have any other questions!