A group of goldfish is called a glimmer, a playful, descriptive term inspired by the way goldfish scales catch and reflect light underwater. Goldfish are prized for their bright orange, gold, and metallic coloring, a trait bred into them over more than a thousand years of domestication from their wild ancestor, the Prussian carp.

When multiple goldfish move together, especially in a well-lit pond or aquarium, the effect really can look like a shimmering glimmer beneath the surface, sunlight bouncing off dozens of scales at once as the fish shift and turn. It’s one of several whimsical collective nouns that have sprung up around goldfish precisely because of their ornamental, eye-catching appearance rather than any particular social behavior.

Goldfish are also sometimes grouped under the terms troubling or simply a school, but “glimmer” remains a favorite for capturing exactly what makes a tank full of goldfish so pleasant to watch.