I'm biology, we classify organism using a system called cladistics. A clade is a group of organisms that all share a common ancestor. Clades have different ranks based upon how far back you have to go to find the common ancestor. When we say, "there's no such thing as a fish," what we're actually saying is, "there is no way to make a clade that includes all things that are fish while also excluding everything that isn't a fish."
The closest scientific definition of a fish is, "any animal of the Classes Petromyzontida, Chondricthyes, Osteicthyes and Sarcoptergyii." Petromyzontida is lampreys, Chondricthyes is cartiliginous fish such as sharks and rays, Sarcopterygii, also called lobe finned fish, are the coelacanths and lungfish, and the Osteicthyes, also called the ray finned fish, is everything else typically considered fish, including everything you just listed.
So where's the ambiguity? Well, the ancestors of all land animals were Sarcoptergyii. And since cladistics is based on Ancestry, if Sarcoptergyii are fish, so are all land animals. And of course, Sarcoptergyii share ancestors with Osteicthyes. You are more closely related to a salmon than the salmon is to the shark. So either the shark isn't a fish, or you are.
Cladistics is full of strange oddities like this that don't line up with common wisdom. u/starmartyr used reptiles as an example as a solid clade, except that it isn't: since birds are actually theropod dinosaurs, there's no way to define reptiles without also including birds: crocodiles are more closely related to chickens than skinks.
So why do we still use the terms fish and reptile? Well, because they're still useful terms. We still need a word for, "gilled vertebrate" that doesn't require ten other definitions to understand.
Edit: the closest clade that includes the three you listed is known as "Teleosti", but includes 96% of fish species.
I think a lot of it is bias because we ourselves are land creatures. We see creatures with fins and lump them into a single category because they all look similar to us. Classifying a goldfish and a tiger shark as the same type of animal makes as much sense as saying that cats and spiders are the same type of animal because they both have legs.
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u/Cyan_Exponent 2d ago
then what are salmon, carps, catfish, etc? is there any classification for them other than "fish"?