Having recently learned about trees, I actually don't think I know what a tree is. at least, not enough to create a reasonable, non-blurry definition of "tree."

You've defined fish as being vertebrates, and as such I cannot find an invertebrate that fits that definition. But what you can't do is just say the word "fish" and expect me to know what you mean - you have to provide a definition, and I could provide a different definition in a different context and neither of us would really be "wrong."

If you did just say "fish" without providing a definition, I would be tempted to either exclude sharks or include crabs, depending on context.