Gustav the Goldfish by Dr. Seuss

A boy overfeeds his pet goldfish, Gustav, despite warnings from his mother. As Gustav grows larger and larger—soon too big for his bowl, then the bathtub—the situation becomes comically unmanageable. Eventually, a solution returns Gustav to normal size, and the boy learns his lesson. Told in rhyme, this early Seuss story (originally published under the pen name Theo. LeSieg) teaches responsibility and moderation in a fun, exaggerated way.