Strange Sudden Aggression In Goldfish

[fishnovice]

One of my goldfish (orange common or possibly stunted koi) recently started acting strangely, chasing many of its tankmates and charging into them deliberately. I've had it since last June and it's never been aggressive before, none of my goldies have. They are a very peaceful community who readily accept newly rescued arrivals of all shapes and sizes.
After about an hour I was concerned it might kill or injure one of its tankmates so I moved it into a tank with a shoal of much smaller golden rudd. It has since behaved like a perfect gentleman with them, absolutely no chasing or bulling.

What made him/her turn agressive towards other large goldfish - mating behaviur or fights over territory? It was the largest goldfish in the tank and I noticed it was only chasing other large common/shubunkin types, totally ignoring tiny goldfish and fancies in the tank.Also seems to be ignoring the rudd now.

The goldfish will soon be moved to their large outdoor tank. Will it be safe to put this aggressive one back with its former goldfish tankmates?

 

[sheppie]

One of my goldfish (orange common or possibly stunted koi) recently started acting strangely, chasing many of its tankmates and charging into them deliberately. I've had it since last June and it's never been aggressive before, none of my goldies have. They are a very peaceful community who readily accept newly rescued arrivals of all shapes and sizes.
After about an hour I was concerned it might kill or injure one of its tankmates so I moved it into a tank with a shoal of much smaller golden rudd. It has since behaved like a perfect gentleman with them, absolutely no chasing or bulling.

What made him/her turn agressive towards other large goldfish - mating behaviur or fights over territory? It was the largest goldfish in the tank and I noticed it was only chasing other large common/shubunkin types, totally ignoring tiny goldfish and fancies in the tank.Also seems to be ignoring the rudd now.

The goldfish will soon be moved to their large outdoor tank. Will it be safe to put this aggressive one back with its former goldfish tankmates?

Hi This sounds like typical spawning behaviour, Its more likely that you have a male chasing females hoping she,l drop eggs known as egg scattering so he can cover them in milt, If you see no sign of it nipping fins and just chasing this is the more likely. Its not uncommon for females to be battered against walls of the tank or pond its just thats how they breed.

 

[Tokis-Phoenix]

How large is the aquarium and what is its specific stocking? Could be spawning behaviour (not going to successfully happen in an aquarium though) but sometimes unideal enviromental factors to can cause otherwise peaceful fish to act out of character. Goldfish are not territorial but might fight for space if the tank isn't large enough.