A group of goldfish is called a sweep, a term likely inspired by the smooth, flowing way goldfish move together through the water. Goldfish are known for their graceful, sweeping tail fins, especially ornamental varieties like the fantail or veiltail, which trail behind them in wide, elegant arcs as they swim.
When multiple goldfish move in the same direction at once, gliding through a pond or tank, the visual effect really can look like a single sweeping motion across the water, fins trailing in unison. It’s one of several whimsical, appearance-based collective nouns that have developed around goldfish, whose ornamental beauty has made them one of the most selectively bred aquarium fish for over a thousand years.
Like “glimmer” and “troubling,” “sweep” is one of the more playful, descriptive terms used for goldfish rather than a scientifically documented behavior, but it fits the elegant, flowing motion that makes goldfish such a pleasure to watch.