I have a 165l tank which has 4 small goldfish: (largest to smallest)1 fantail -possibly male - has dots on gill plates, 1 black moor, 1 rusty coloured moor, 1 panda moor/butterfly. Also have 5 gold medakas & few ramshorn snails.
This tank is fairly new (set up begining of May).
Have had 3 of the fish since early March in a 40 l tank no problems.
Rusty moor was new fish added to big tank by itself 13/5. Other 3 were added progressively (smallest to largest) over week from 28/5 to make sure they socialised ok. They've all been in there together for over 2 weeks and there's been no problems noticed until today.
This morning the fantail was hassling the rusty moor badly pushing it around into plants & tank ornaments so much so I thought it would injure it. The other fish also joined in on the bullying. I was so worried I took the rusty moor out and put it in a small emergency tank. As soon as it was gone the other 3 went back to normal foraging in the gravel and behaving as they always have. After about an hour I hoped they had calmed down as this behaviour was out of the ordinary, I put the fish back in, but it started again straight away. So out the poor thing came again.
Does anyone have any ideas why this behaviour has suddenly started? Could the rusty moor be female & the others male? Am I going to have to keep them seperated permanently? I really don't have room for another goldfish sized tank. Plase help.

Thanks for your help.
You are most likely right but I'm in Australia & its winter here so shouldn't be spawning season.  Although our winters are pretty mild compared to some places if they spawn in warmer temps what temp triggers that behaviour? I have a themometer in the tank & the temp is a fairly constant 22C (71F) & not likely to get any colder. Our winter temps here are approx 18/16C max - 6C min.  Would this mean he will be at it all year?

Also the other 2 fish in the tank, neither are showing any signs of tubercles to indicate they're male yet & one is larger than the rusty moor he chases. Does the fact that he leaves them alone indicate that they're immature males? So if I were to seperate the fish would it be best to move the rusty moor female?

Thanks for your help.
I've had the female out of the tank for over 24 hrs now (she is easier to catch!). I'll try her back in tomorrow but if our fella plays up again it's his turn in the sin bin! (if I can catch him).

How long does it normally take for males to get out of breeding mode?

I'm trying to avoid getting another tank as I don't know where to put it & don't really want to have to maintain another one. Already have 4! (2 x male bettas in 20l; 40l - old goldie tank- with guppies & platies plus this goldie tank). Can't give the troublesome fish away as they are my daughter's babies & she would be heart broken.
Who said fish were easy pets!?!

LOL, nobody said they are easy pets, rofl, WELCOME TO MTS ( Multi Tank Syndrome ) we've got 8 tanks bwaahahahahah

You're right about the pet store staff, most likely in bigger chains. But in small privately owned stores it is just a joy to go there.

And surely you are not a worrywart ( LOL ) you just want to know about things and explain it accordingly to your daughter. BTW I send you a message in Aqu. DownUnder 

Hanna: Thanks, I didn't know that!  When a red cap oranda of mine got breeding tubercles once, they seemed to leave completely after about half of a year. However, he was white, so....  kinda hard to tell.

Like Nossie said, the other smaller fish are probably just bullying...not spawning. I think the one with breeding tubercles probably is trying to, however, but that's just my opinion, listen to the wiser ones on this forum, not me 

It's easy to end up biting off more than you can chew when the petshop's staff brings you malinformation. The staff around where I live says that a goldfish will do fine in a 5 gallon tank, so that shows how much they reallly know about fish 

Great to hear! Keep us posted.