In all seriousness, one of the two words “fishes” and “wordism” is made up, and the other is not. If you asked people, I would say most of them would single out “fishes” as the impostor. And most of them they would be wrong. To be perfectly clear, “fishes” is a word . However, it doesn’t mean exactly what you’d expect. Hell is only slightly cooler than usual, and the plural of “fish” is still “fish”. However, in case you would like to specify that the particular group of fish you are talking about consists of more than one species, you may use the word “fishes” instead. The examples on the Wikipedia entry do a good job of explaining the difference.

The North Atlantic stock of Gadus morhua is estimated to contain several million fish.

My aquarium contains three different fishes: guppies, platies, and swordtails.

The former is a regular old plural; there is more than one fish. The latter is a metaplural, if you will, and its plural-ness refers to the types of fish, not the fish themselves. Another metaplural that is in much more common usage is “peoples”. This word refers to different types (ethnicity, race, etc.) of people, a word that, like “fish”, is already plural. My favourite example is “The Peoples of Middle-earth”.

Update: This might be easy to understand, but harder to believe. Perhaps the Australian Museum has more credibility that an unknown blog and a piece of user-generated content.