Fan CastingHelp! I'm a Fish

The film follows the adventures of three children: a skateboarding mischief-maker named Fly, his sweet younger sister, Stella, and their cousin Chuck, a cautious, intelligent and overweight genetics prodigy. When their babysitter, Aunt Anna, falls asleep, the three children sneak off to go fishing only to stumble across the boathouse home of Professor MacKrill, an eccentric marine biologist. Reasoning that climate change could melt the polar icecaps within the next century, MacKrill has developed a potion that turns people into fish so they can survive the rising sea level and also an antidote to reverse the process. Unbeknownst to all, Stella drinks the potion mistaking it for lemonade, and painfully transformed into a starfish and gets tossed out of the window into the sea. Since Stella's transformation was caught on camera, the tragedy is immediately discovered, so Fly, Chuck and Professor MacKrill head out onto the ocean in a desperate search. When a storm blows in, Fly recognizes the futility of their search and goes to drink the potion. Though the professor warns him that if he does not get the antidote within 48 hours he will be a fish forever, Fly drinks it and jumps overboard, becoming a "Californian Flyfish". The boat capsizes and, because Chuck cannot swim, he's forced to drink the potion to survive, becoming a jellyfish. The Professor, the boat and all of its contents sink beneath the waves. A great white shark and a pilot fish come across the leaking bottle of antidote and gain the human characteristics of speech by inhaling the liquid. Using his newfound gifts, the pilot fish, who now calls himself Joe, sets about creating an underwater civilization of intelligent fish. They take residence in a sunken oil tanker and begin to transform it into a monument. Fly, Chuck and Stella are reunited, along with a seahorse named Sasha, but are horrified to discover that the antidote has been lost. If they don't find it before tomorrow's sunset, they will stay fish forever. Some traveling fish tell them about Joe and his "magical potion". Thinking it must be the antidote they are looking for, the children travel to Joe's oil tanker empire. At the tanker, Fly attempts to steal and drink the bottle of antidote, but is warned by Chuck that if they turn back into humans this far beneath the ocean, they'll die. Intrigued by this, the villainous Joe has the children arrested and demands they manufacture more of the antidote or he'll have them eaten by the Shark. Meanwhile, Fly and Stella's parents, Lisa and Bill, arrive home to find Aunt Anna frantic with worry. They find that Fly's fishing equipment is gone, so they head to the beach to search and come across Fly's roller blades instead. There, they meet Professor MacKrill who, having survived the storm, explains that their children have been turned into fish. Though the parents are skeptical at first, a showing of the video recording from earlier validates his story and causes Aunt Anna to faint. He and Bill head out to search for the children in a cobbled-together ship fitted with a large water pump. The next morning, the children manage to escape with the help of Sasha. With no chance of going back to retrieve the bottle of antidote, they decide that their best hope is to find the ingredients to recreate the antidote themselves. Just as they complete the formula, they are found by Joe, the Shark and their army of crabs. During the standoff, Joe and the Shark get into a heated argument as Joe imbibes more of the potion, developing hands and growing in size. Taking this opportunity to escape, the children are stopped by the leader of the crabs who attacks Fly, striking him with his claw, then drinks the antidote himself, growing in size and developing hands and feet. Just as the new "King Crab" and his army are about to capture the children yet again, a tremendous underwater twister, generated by the Professor's water pump ship, sucks all the crabs (and the Shark, who eats the King Crab in the middle of the twister) to the surface. The Shark remains stuck in the tube. Now alone, with Fly dying, and only twelve minutes until sundown, Chuck realizes their last hope is to make it back to the Professor's lab, where a whole jug of antidote is stored. Showing unexpected courage and determination, Chuck carries both Fly and Stella through the dangerous seawater intake pipes back to the lab. However, they are pursued by Joe, who overpowers them and steals the antidote. While Chuck fights off the Professor's escaped piranhas, Fly manages to catch up to the fleeing Joe as he escapes into a water intake pipe and tricks him into drinking more and more of the antidote by asking him various questions on science. Joe drinks enough of the potion to render him more human than fish, causing him to drown. Fly drags the jug of antidote back to the lab and collapses. Chuck uncorks the jug just as Lisa and Aunt Anna open the door to the laboratory, causing everyone to get swept away. As the water drains away, Chuck and Stella have become human once more and are reunited with their parents. After a few tense moments in which a stuffed fish is mistaken for the limp body of Fly, the human Fly emerges from one of the lab's pipes (with a broken leg). Later on, while playing by the beach, Stella is reunited with Sasha, who is turned into an actual horse by the Professor and Chuck. We zoom out to see the beach with Stella riding Sasha in horse form as the film ends.