Fantail goldfish are some of the best species of fish for beginners looking to establish a home- or office-based aquarium due to their long, flowing fins, hardy constitution and general ease of care, according to ExoticGoldfish.net. They are generally a peaceful breed but certain circumstances can bring out their aggressive side. Hunger, stress and a desire to mate can incite them to chase each other in a quest for dominance.
Male fantail goldfish will chase females during mating season as part of their courtship ritual. According to FishChannel.com, pheromone chemicals are released into the water when mating begins, sparking the chasing frenzy. At first, all fish may chase others of both genders, particularly if they are immature. But, eventually, nature sorts out the correct mating pattern and you will be able to to distinguish which of your fish are males and which are females.
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. General recommendations indicate that a 20-gallon tank is suitable for one fantail, and 10 gallons should be added for each additional fish.
Introducing a new fish into an established tank can also trigger the intrinsic territorial nature of your fantail goldfish. The new addition may become the target of the chase during the first few days after he enters the tank. Like any other animal, some fantail goldfish are simply prone to bullying and you should pay close attention to see if this behavior lets up over time. If it doesn't you may have to isolate the aggressive fish by using a tank divider or by placing him in a separate tank altogether.
Goldfish are social animals and often do best when housed with a companion. Like any other species they interact with each other in a variety of ways. Chasing each other can simply be their way of playing when bored. If the chasing is not always one-sided and doesn't result in nipping or any injuries it is most likely just for fun.