Why Are All My Goldfish Chasing One Fish?

Overcrowded. Fantail goldfish can become aggressive if they don't have enough space in their fish tank. According to PureGoldfish.com, confined quarters can make the fish feel like they need to compete for space, causing them to chase one another in an attempt to claim their territory.

Why are my goldfish attacking one fish?

Territorial

The most common cause of a goldfish being territorial is if they are jealous of other fish in the tank or if the tank is too small. Territorial behavior is distinguished by chasing and nipping other goldfish away from a certain area in the tank. Solution: Provide a large living space for the goldfish.

Are my goldfish fighting or mating?

The mating “dance” of goldfish is tiring and they will chase each other nearly to the point of exhaustion! This usually goes on for several hours and may also include the male nipping at the female's tail and fins. What is this? For obvious reasons, this behavior is often confused with fighting.