How to Keep Goldfish From Biting Each Other

Most goldfish live together peacefully with few outward displays of aggression. Biting behavior typically indicates a larger cultural problem that requires treatment for both the victim and the aggressor's health. Fish suffering from attacks exhibit ragged fins or missing scales. The victim fish might also grow more slowly if the aggressor is chasing it away from the food at feeding time. Watch the fish closely for signs of biting and act quickly to prevent serious injury or death.

  1. Tank Concerns

    • Goldfish require at least 10 gallons of water per fish. Goldfish longer than 2 inches might require 15 gallons each. Placing two fish in an overly small tank can result in aggressive behavior such as biting. Poor water quality can also cause biting. Perform a partial water change on the tank weekly by replacing half the water with fresh. Provide real or artificial aquarium plants and decorations so the fish have a place to seek cover from each other during short attacks.

    Fish Types

    • Goldfish come in two main varieties. The single short-tailed types have shorter tails and fins. The fancy double-tailed and fan-tailed varieties have long, trailing tails and fins. Single-tailed types include the comet and some shubunkin varieties, as well as the most common feeder varieties. The broadtail moor and glob eye are examples of double-tailed goldfish. Single-tailed goldfish are attracted to the long tail and fins of the double- and fan-tailed types, often biting and sucking on them. This eventually damages the tail and can cause fatal injury. Attacks are even more likely if the double- or fan-tailed fish is much smaller than the other one. Biting might also occur between two goldfish of the same variety if a large disparity in their size exists.

    Preventive Measures

    • Feed the fish at the same time daily. Supply only the amount of food that the fish can eat within 5 to 10 minutes. Under-feeding, over-feeding and missed feedings can cause aggression. Separation, even for a brief period, can stop biting behavior. Place a clear acrylic tank divider in the aquarium to separate the fish. Use the divider temporarily to stop aggressive behavior until the fish are of equal size, or use it permanently if aggression continues.

    Diseases

    • Study the fish for signs of disease, which can weaken a fish and makes it a target for aggression. Red gills, ragged fins or a white growth on the scales and fins all can signify illness. Quarantine the ill fish in a separate tank or bowl while you diagnose and treat the problem. The fatal fish disease Ichthyophthirius multifiliis, or ich, caused by parasites, covers the fish with white spots and causes the fins to look ragged and chewed on. Early symptoms are sometimes mistaken as biting damage. Treat ich immediately with a medication formulated for the disease.