This has nothing to do with fish. 'It was a red herring' is an idiom, referring to a piece of irrelevant information used to deliberately to waste time and lead a line of enquiry in the wrong direction. For example, if you are reading a murder mystery, 'a red herring' is evidence suggesting that person A might be the murderer when in fact it was person B.

A red herring can be fish, but its also an expression that refers to an argumentative fallacy. In argument if someone tries to throw something into the argument to distract the opponent from the real issue he/she has thrown a red herring.

It's a type of fish :)