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?

 

[Baccus]

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?

It sounds most likely to be breeding behaviour. Was it mostly targeting plumper goldfish in the group? I would say that once out in to a large out door tank the chasing will continue if it is because it's wanting to breed. As long as the chasing isn't too aggressive and the other goldfish aren't getting totally pummeled by the male (?) it should all calm down once spawing has been acheived. If you want the eggs and fry to survive I would make sure you have either some clean mops heads to suspend in the tank or plenty of resilient water weed that can stand up to the goldfish pushing through to lay their eggs in the tangle of weed. Water hyacinth and Water cabbage or great as they have nice fiberous roots for the goldfish to lay thier eggs in, plus you can then remove the enitre plant(s) and hatch the eggs away from the parents.

 

[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?

It sounds most likely to be breeding behaviour. Was it mostly targeting plumper goldfish in the group? I would say that once out in to a large out door tank the chasing will continue if it is because it's wanting to breed. As long as the chasing isn't too aggressive and the other goldfish aren't getting totally pummeled by the male (?) it should all calm down once spawing has been acheived. If you want the eggs and fry to survive I would make sure you have either some clean mops heads to suspend in the tank or plenty of resilient water weed that can stand up to the goldfish pushing through to lay their eggs in the tangle of weed. Water hyacinth and Water cabbage or great as they have nice fiberous roots for the goldfish to lay thier eggs in, plus you can then remove the enitre plant(s) and hatch the eggs away from the parents.

Thanks for your reply. All my goldfish are well fed(overfed?) and are beginning to look like heavily pregnant guppies! Yes he was indeed chasing only the largest/fattest common type goldfish, ignoring fancies and babies.

I had heard that goldfish mating was pretty rough but I was afraid it might seriously injure or kill the others if it was a bully. How much violence and aggression is normal/permissable in goldfish before it reaches danger levels.If this behavious is normal I might put him in with the others at Easter and try to breed them. It would be fun to have baby goldies.How do you keep a mop floating in the tank, sure it is too heavy and will sink?

 

[Baccus]

The worst danger I think occurs when you have more males wanting to breed than available ready females, that and tank area. If the females can get a break from the males pursuits everything should be ok. Goldfish start breeding when they have a combination of increased daylight and warmer water temps, well that is how it works in ponds so I guess the same could be said for a tank. If you wanted to postpone any breeding, until you are ready with extra space etc for any fry, you could try reducing the amount of light getting to the tank and keep the water a bit cooler to mimic it still being winter. Where I am in Australia my goldfish/ comets almost always breed in September in their pond as around this time the sunlight starts getting through the surrounding tree foliage and the light hits the surface of the water earlier in the day also the water is just starting to warm up from the cooler winter temp.

The type of mop head to use is one of those cotton multy strings types. Just get the plastic end part that is supposed to attach to a mop handle and put some wire or a bulldog clip to anchor it to the side of the tank. Once fully saturated some of the cords should sink while others will probably want to foat on the surface. If you wanted to you could attach/ tye into the cords some small pebbles or something with a bit of weight to get the cords to sink. Alternatively you coud try attaching the plastic end part to a rock or heavy tank ornament and have the mob head keeping itself vertical in the water. Either the goldfish will almost certianly use it.

You could also put a whole heap of Elodia or similar weed in the tank with the goldfish just leave it floating as a tangled mass and they will spawn into it. If you tried palnting it the goldfish will almost certianly end up just tearing it out with all their breeding activity. Be warned if putting the plants in the tank be prepared for the goldfish to have a good feed on it as well as breed in it.

Hope this helps.

Oh and since you have the goldfish in a tank you should be able to sex the mature ready to breed males by the white tubercles that will appear behind the males eyes and around his gills. I have never really noticed this with any of my fish as they are always in their pond and most of mine are predominatly white so I would have to do some serious scrutinising of the male in question to find the spots. I will post the only half way decent pic that I have of the male tubercles. Seem to be having trouble getting the pic from my files to PhotoBucket so I'll have to try again later to figure it out.