In the book, "funny things are everywhere," and the characters accept all the bizarre and strange-looking creatures because, after all, they are all beloved pets no matter their appearances. You might be wondering why these kids are so welcoming, but it's all right there—at the beginning.
See, there are red fish, blue fish, old fish, new fish—every kind of fish you could possibly drum up in your noggin. But at the end of the day, what are they? Fish. They're all fish—no matter their size, shape, or color.