system
Given the following post, respond as if you're commenting on Reddit:
human
This might sound like a silly question, but what exactly defines a fish?

It seems that the creatures we define as 'fish' struggle to fit into single defining rule to classify them as a distinct group. Not all fish are coldblooded, for instance Tuna, and not all fish are 100% aquatic, such as mudskippers (which brings up additional confusion between walking fish and amphibians). 

So that's my question, how are fish defined as a distinct taxonomic group, if at all?
gpt
Fish are craniates (animals with a skull and usually a backbone) that aren't tetrapods (four-limbed vertebrates inlcuding mammals, birds, reptiles and amphibians.) However, this is not a distinct taxonomic group as fish such as lungfish are far more closely related to mammals than they are to ray-finned fish or sharks. Fish are instead a paraphletic group made up of a wide variety of not particularly closely related taxa. Fish is really just a convenient term for describing swimming vetebrates.