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You can absolutely train fish! (And crabs, and octopi, and sheep, and cockroaches...)
Here's another example video in which Karen Pryor (the "mother of clicker training") teaches an Oscar (fish) to swim through a hoop: Reaching The Animal Mind (it's the first video link: Fainting Fish).
Karen also trained a hermit crab to ring a bell (by pulling a string), and recently a high school student replicated the experiment. It didn't take long, either.