All fish are not phylogenetically correlated:

Things became fish multiple times independently.

There is no "first fish from which all fish derived".

Phylogenetic existence refers to the evolutionary history and relationships of a species as represented in a phylogenetic tree. This tree is a diagram that depicts the lines of evolutionary descent of different species, organisms, or genes from a common ancestor.

So monkeys are phylogenetically related, because all monkeys that we know have common ancestors.

Fish came to be multiple times independently. Being a fish, a tree, or a crab is a strategy, not a species.

Which is ironic because we call it the "tree of life", but it should be "forest of life" (but since life originated in the sea, it should be the "sea of life"), since trees don't have a single phylogenetic root: There wasn't a "first tree that all trees descend from": Things became trees independent of one another, because being treelike is beneficial early on, much like being fishlike and crablike.

I understand the idea, I was asking for a link where the original "there's no fish" is discussed. I've seen it linked on HN but haven't been able to find it since.