Here is a cute problem that requires visualizing two-dimensional shapes within a three-dimensional object, and using properties of triangles and the Pythagorean theorem to solve a real-world problem. There is a small mistake in this problem, that your charges are sure to let you know about. Clown fish are not goldfish, and two couldn't live together in the same fish tank. If this fact becomes an obstacle, then challenge them to rewrite this problem in a totally different way.