Why Does One Fantail Goldfish Chase the Other?

Fantail goldfish are named for their large, fanlike tails. They come in a variety of color patterns ranging from white to red and prefer cool water temperatures. Though these fish are typically friendly and nonaggressive, inappropriate environmental conditions may cause one fish to repeatedly chase and even attack another. In other cases, the chasing behavior may be perfectly normal. If your goldfish are chasing each other, closely observe the behavior to determine the cause.

  1. Mating Behavior

    • During mating season, male goldfish frequently chase females as a part of their mating ritual. Goldfish normally breed only when there has been a sudden temperature change in the water. If you've recently lowered or raised the temperature in your aquarium, you may have stimulated breeding behavior in your fish.

    Overcrowding

    • Goldfish require 2 gallons of water for every inch of fish. If you have two 5-inch fantail goldfish, for example, you need 20 gallons of water. Fantail goldfish become aggressive and territorial when their tank is overcrowded, and may chase and inflict injuries on each other. Invest in a larger tank or buy additional tanks and move some fish to the extra tanks.

    Nutritional Deficiencies

    • If your tank is not overcrowded, check your goldfishes' diet. While fantail goldfish will always eat if given food, they won't chase and bite other goldfish due to hunger unless they are suffering from a nutrient deficiency. Give your goldfish as much food as they can eat in 15 minutes every day and make sure all of your goldfish are eating. Invest in a high quality goldfish flake or pellet food.

    Play

    • Goldfish, like most other animals, sometimes play with one another. If no one is being injured during the chasing sessions, the fish may simply be doing something that's fun. Watch your fish to see if they occasionally switch roles, with different fish being chaser and chasee. If this is the case, you don't need to intervene and your goldfish are unlikely to injure one another.