I don't have a clue how she managed to get it on there, but seconds after I took the picture, she thrashed around and cast her wafer hat off in what I can only imagine was disgust at being caught with it on her head.
For reference, it was an Xtreme Cat Scraper....I'm not sure if I should take this as a vote for or against them....
I think I saw references to Oolong on another forum I frequent, not fish-related. Apparently it was once commonplace to reply saying "I have no idea what you're talking about, so here's a picture of a bunny with a pancake on its head."
Maybe we can use your picture similarly: here's a fish with an algae wafer on its head.