I have had this happen also (not goldfish). For no reason sudden aggression. One method is to lower the water temp. If the water temp. has lowered(very hot this summer in NJ) that has been said to be a cause. Other suggestions are diet, amount of food. Anything different with the type or kind of food you are useing. Some times as fish get bigger and older thay get a little aggressive,especially if tank mates are much smaller. The space sounds good for both(I am Not sure of what a lionhead is I guess its the same size as the Oranda?) I guess try to lower Temp. if it has climbed, If it doesn't work then seperation is suggested. The stress will cause the bullied fish to get ill or die. I never had luck totally lowering the temp. I keep tropicals and you can only go so low. In the summer when the lights are on for the plants I did notice more aggression. I suspect since you have goldies you have no live plants. Try directing cool air towards the tank with a fan or A.C. unit / Exchange some heated water with fresh cooler(add the cooler water slowly so not to make an abrupt change)/ Some even float ice cubes in severe heat/ a chiller (very expensive)/ I had an idea for a DIY water chiller but I haven't found a controller yet that will turn off and on a pump that is inexpensive enough. Good Luck

woeisme....I think you may have nailed it. First thing this morning when I checked on them to feed them, the 2 of them were swimming "peacefully" in the tank. Their tank light had been off and the room only had natural daylight filtering through. As my habit is to turn light on first thing, I did so & proceeded to feed them. Quickly it hit me that maybe I'll keep this light off since maybe it is more soothing having dimness. When I read your response it definitly could be the very warm weather we are experiencing here and then along with me keeping their tank light on, it must be warmer in there than what they are accustomed to and is highly aggitating the big fish. I do notice when I clean their tank & they get their partial water change (fresh cool water) that they are very peacefully and gracefully swimming, so there definitely must be a connection. My Lionhead is about 2" compared to my 6" plump Oranda so there is a major difference in size but they are both from goldfish family. I've had both for 2 years now & this aggression has only happened in past week or so (coincides with hot weather). I thank you so very much for your help. I will keep my fingers crossed that keeping them cool is the answer. I dropped a couple ice cubes in and I am going to set up a fan nearby to keep the immediate vicinity of the tank on the cool side.

melle Sacto....I took your suggestion and looked for breeding tubercles and could not see any on the fish. Thank you for letting me know about this because I will keep it in mind to watch for this sign. Thanks again both of you for taking the time to help me.

Greenisland,

A rise in temperature can also cause the fish to breed. The
breeding tubercules don't always show. Goldfish don't get aggressive unless they are in breeding condition. And the actions that you discribe are breeding actions. This chase may continue for several days....off and on. There will still be some peaceful moments like you mentioned in your second post.

I will definitely be keeping very close watch on the "happenings" in the next several days. I am a newbie with goldfish (I formally had only tropical fish) so any helpful advice on the Oranda species is very much appreciated.

Hi, All goldfish are Carassius auratus, so Oranda is not a species. It is just a type of goldfish.

Not sure what else you would like to know about these fish....if you ask more specific questions..I may be able to answer.

I had the same problem with my Central American Cichlids, especially the Oscars, who literally bullied and tried to eat anything they could get in their mouth. I did what the so-called experts and written word said, by putting them together with compatible tank mates. My Jack Dempsey gave as good as he got from those Oscars though, in the end, I had to separate into separate tanks or risk losing my babies.

But, I tell you, their was more action in that aquarium, than fight night. I personally feel sorry for the female Oscar during mating season, they get the heck beat out of them, all in the name of "fish love."

Flo

The aggressive one is the male which is in mating condition. It will do a lot of chasing and will bump its nose under the other fish if it is a female to encourage it to drop its eggs which the male will fertilize with a milky solution from its organ below it. I have had this happen many times and the plants in the tank will be scattered with clear egg cells sticking to them. In a few days time, the baby gold fish will hatch and wriggle around the tank looking for food and somewhere to stand as they can't swim yet with hardly any fins. Remove the adult fish as soon as the eggs are seen or they will swallow all of them up quickly. Feed the babies with solution food for a few days and when they get a bit bigger, use live brine shrip or other fine live foods. Welcome to correct me if you think I am wrong, cheers.

I have noticed this behavior in my two fan tails. One is about six inches the other about three. The bigger bullied the smaller so much he got swim bladder disease. I separated them and when SBD went away put them back together. It was only then I noticed the aggression of the bigger. He kept pushing the little guy against things, coming up underneath him and biting. I have looked for the sex indicators but can't pick them out. I don't know if I am going to be expecting or if the bigger of the two is just a jerk. My little guy is missing some scales as well and I noticed an abdominal fin has been chewed off but there are absolutley no other signs of illness. I've googled everything. My tank water sits anywhere between 72 and 76 degrees at all times and for the last five months we've had them there have been no other issues. They got along just fine. I guess the question is would a male be aggressive enough to a female to chew fins off? He leaves our little catfish bottom feeder alone. I also read to put more hiding spots in so I plan on adding a ton of plants during the next complete water change. We tried live plants but they got so expensive as the big one kept eating them all!

I have one male lionhead about 2 and half years old in my fish tank.... and he always attacks other gold fish in the tank... he attacked a male fantail thats over 6 years old and he also attacks oranda goldfish that are almost the same size as the lionhead.... but i have no idea about the genders of the oranda fish.... so why will my lionhead attack other fish ???

Hi. Guys..we have an oranda goldfish with pearlscale goldfish..the first one is female and second is male because they mate many times these days and we have raised their babies too..about 500 kids after one month are alive in seprate aquarium..my female(oranda) sometimes attacking male and when it's time for her to release her eggs the male chasing her smoothly..but i am worried for male because he is smaller and can't escape from her soon so his scales has fallen and if she continue other scales may fall too:( as i got, orandas are very naughty,anti plants, greedy for food,and they are good fighters too! I like her very much but I wished she didn't attack the male

I have one calico Oranda in a tank with one smaller black moor fish. The calico is exhibiting exactly the same behavior you described. Also it is larger than the black fish so I am concerned about the black one being able to get enough food. Calico acts like a little piglet. Don’t know what to do to protect the black one. Anyone else know?

WOW! this is all so interesting for a novice aquarium owner. I have a black moor fish with a larger calico Oranda one. The calico is very aggressive at feeding times and periodically at other times. May have fish eggs too. What color are they? Do they clump together? Something white has clumped onto the pipe or stem of pump in water. Oranda acts like he would eat til he explodes! I did see a milky substance come from one of them a few days ago. Can anyone tell me what is going on?