Something fishy…
McDonald’s appears to be the subject of more mythology than the Greek god Apollo. Remember the rumor about the complete lack of milk in the shakes? Not true. Yet your question remains: is there something fishy about the fish?
An ingredient list at McDonald’s specifies the following for their “Filet-O-Fish”: fish filet patty, bun, tartar sauce, pasteurized processed American cheese, and seasoning. Still, any product with such a poor spelling of “fillet of fish” bears further scrutiny.
A good morning read!

Actually, according to hyperdictionary.com’s definition, either spelling works. I always thought the second L was superfluous myself.