Spawning goldfish can be insanely hostile. The way to distinguish spawning aggression from other aggressive behavior is to first see if you can spot the fish’s genders.
2. Feeding Jealousy
The way you determine if it is feeding jealousy behavior is that you’ll notice they only (typically) start this behavior at mealtimes. It’s also not typically as fast-paced as mating behavior.
3. Territorial
A fish that’s had it’s home all to itself may not be happy with a new invader in its space, so they try to show them who’s boss.
4. Sickness
One thing about goldfish is they don’t always show a lot of compassion. When one is sickly or weak, sometimes the others make the situation worse for it and start attacking or pecking on it. Sad, but it happens.
5. Higher Stocking Densities
Sometimes, fish that are kept in more crowded conditions for a longer time may be more prone to picking on each otherю.