The smallest fish I've ever seen or eaten, at least in their dried state which is how they are usually sold, are 丁香鱼 (dīng xiāng yú), clove fish or, less often, lilac fish.
Always eaten as a condiment, these tiny members of the herring family are native to the East China Sea where they live on plankton. Dried, they rarely exceed 3cm in length.
They are often used in Chinese XO sauce where they bring umami, are added to congee or fried with vegetables.
My favourite is this bottled version with fermented soy beans. Salty, savoury and umami packed they make great beer food.
