Goldfish are normally placid by nature. Aggressive behavior, while not the norm, is not uncommon, however. This is most often manifested by stalking behavior and/or fin nibbling, which can lead to bacterial infection. What causes this behavior? How to reduce aggression?
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Can goldfish kill each other
99.9% of the time, a goldfish will not kill for no reason, however, a goldfish may attack another goldfish if it is smaller than him. It will then see it as prey, but the intention is not to kill, but to feed.
How to know if my goldfish is fighting or playing
As a general rule, it’s pretty simple to tell if a goldfish is aggressive or not. They don’t have much interest in fighting, however, they have no qualms about devouring a fish smaller than themselves, so if one or more of your goldfish are following a smaller fish around all the time, they are trying to eat it.
Why do goldfish fight each other?
Overcrowding (lack of space)
Make sure your fish have enough space. In a small space, all goldfish, including Japanese varieties, can be aggressive.
Poor water quality
Often related to an overcrowded tank, polluted water can reinforce aggressive behavior. Check your ammonia and nitrite levels (see emergency solutions here). In a properly filtered aquarium of adequate volume, ammonia and nitrites are undetectable with commercial tests.
Mixing slow varieties (pearl, celeste, fan tail, etc) with more “dominant” varieties (common goldfish, Ryukin, etc). This, especially in small volumes, can create problems of aggressiveness and unequal access to food.
The breeding period
During the breeding season, this type of behavior is common, whether it is the males parading or the females trying to protect their eggs from predation. Note that an insufficient number of females compared to the number of males is also a possible cause.
Sufficient space with decorative elements and plants can provide retreat spaces, limiting eye contact. If this is not enough, temporarily isolate the overly “fiery” male.
Quite often, a sick or weakened fish will become the victim of the group. This is due to the survival instinct, as the disease can spread to the rest of the group, and the other fish will try to get rid of it by harassing it; however, it has nowhere to go in a closed space like the aquarium. Installing a quarantine tank will allow your sick fish to rest and be treated effectively.
Lack of food
This is rarer, as goldfish are more often overfed than underfed, but it is still an avenue to explore.
How to limit goldfish aggression
- Provide sufficient volume and swimming space for your fish
- Do not overcrowd the tank.
- Maintain a good water quality
- In terms of aquarium maintenance, make regular water changes.
Keep each variety (or compatible varieties) in a specific aquarium
Do not mix fragile varieties (pearl, celestial, fan tail, Blackmoor, Pompom, etc.) with more vivid varieties (common goldfish, comet, Sarasa, Ryukin…)
Install decorative elements and plants, which provide hiding places for fish.
Ensure fair access to food
Two reasonable feedings per day are recommended. Observe your fish and make sure that all fish have access to the food during feeding.
Distribute sinking pellets in two different places
Plants can also be a welcome snack and keep your fish busy. Quarantine your fish if they are sick.