HAS ANYONE NOTICED THAT ALL OF THE SPECIAL PEOPLE ARE SOMETHING BESIDES FISH?

Yeah , That's true. THat's probably because those who you mentioned are the most important and people mostly pay atenttion to them , not to others, or the background. And , while making a cartoon, It's probably hard to make all characters DIFRENT and unique , because that takes a long time. Why to make background characters unique , when people won't look at them anyway ? ( sorry If I spelled something wrong )

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because it would'nt be cool if the main characters were fish like the others bacause it would make them boring.so instead they make them other sea creatures that are not normal fish.

Mrs. Puff is a fish.