What are false fish in a tank?

"False fish" is not a standard term used in the aquarium hobby. It's possible you're referring to one of the following:

1. Fake Decorations:

* Plastic Plants: Many aquariums use plastic plants for decoration. They are often used to mimic real plants and provide a more natural look.

* Decorative Objects: Other common aquarium decorations include castles, rocks, shipwrecks, and other objects that are not living. These are made from various materials like plastic, ceramic, or resin.

2. Fish that are Not Actually Fish:

* Invertebrates: While not fish, some invertebrates like shrimp, snails, crabs, and crayfish are common inhabitants of aquariums.

* Amphibians: Some amphibians, such as frogs and newts, can also be kept in aquariums.

3. Misidentified Fish:

* Sometimes, fish are misidentified due to inaccurate labeling or lack of knowledge. This could lead to someone thinking they have a certain type of fish when it is actually something else.

Please provide more context or details about what you mean by "false fish" so I can better understand your question.