I've made an aquarium and a couple ornamental ponds, and I've noticed that sooner or later, schools of fish get stuck near a wall. It seems that one fish fails to notice that it's pressed up against a solid block, and all the fish schooling with it just follow suit. Pufferfish are less prone to it since they don't school, but I've noticed one of them doing it at least once. Other aquatic mobs appear to be unaffected.