These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Fantastic Racism: The reason Selkie reacts so poorly to fish-related insults is because she's an obligate carnivore from a predatory species whose main food source is fish. To be called a fish is an implied threat, with the aggressive party calling you weak, helpless, and only good for being eaten.
Fridge Horror: An In-Universe example here, when Theo pieces together the various hints that Agent Brown had dropped regarding Selkie's people.
"Funny Aneurysm" Moment: It hasn't happened yet, but it's been set up. Amanda snarls at Selkie for "stealing" a daddy from her, since Selkie got adopted and Amanda didn't, triggering serious resentment. "People would rather adopt a fish than me." It has since been revealed to the audience that Amanda is Todd's biological daughter, but Todd and her mother Andi were teenagers at the time and Andi didn't feel ready for/didn't want kids, so she gave Amanda up at birth before Todd even got to the hospital, and told Todd that the baby had died/was stillborn.