鱼儿 is the colloquial form of 鱼 (fish), formed with the affectionate suffix -儿 (ér). It carries a softer, more endearing tone than 鱼 alone and is commonly used in children's stories, songs, casual speech, and when talking about fish in a fond or playful way. Though it refers to the same animal, the suffix makes it sound cuter or more familiar.