Hi. I've just joined the forum. My apologies for the rushed nature of this post. I need to get some expert advice ASAP and once I have set the ball rolling I'd like to introduce myself properly on here! So, straight in at the deep end...

I have an established 7ft setup with 1 red tailed shark, 2 silver dollars (5"), 3 gourami's (3"+), 1 Giraffe Catfish (14") and one Plec (12"). I was put off adding Tiger Oscars to the tank for some time by many LFS's but I was at a LFS the other week and was talked into buying two 3" Tiger Oscars on the promise that they woud live (and I quote) "happily with my current fish". I also purchased three Jewel Cichlids. Now I would like to add I am reasonably experienced in fishkeeping and so this was not an impulse buy at all. Anyway, onto the problem:

Basically for the first 2 weeks all the fish lived in perfect harmony. Suddenly overnight 2-3 days ago, one of the Oscars has turned VERY agreessive. First, he has chased and caused a major amount of damage to the red tailed shark. He may not survive it I fear but is still alive right now. Next, he's (or she) moved onto the Gourmi's - bullying them 24/7/365. He won't leave them alone. The other Oscar is fine and very chilled out and they get on well together, rarely leaving each others side.
I do not understand what has changed. Anyway, I'd love to get any advice on any tips to stop the bullying without having to remove either the Oscars completely and give them away, or move the bullied fish to a smaller spare tank, which quite frankly would not be fair on them as they have been here for almost 3 years.

p.s - the "trouble" Oscar seems to have also got white spot at the same point as turning nasty. Is this coincidence? I am going to follow the forum notes on removing the whitespot so I hope that works. COuld this be what has turned him all aggressive?

Looking forward to chatting to you all. I really do love my Oscars and want to get the best out of them.

Welcome to the site. I don't think anyone can really answer your question to why this has happened. Who knows why these aggression problems occur and why some Oscars are more aggressive than others? I have been lucky enough to have Oscars that don't have aggression problems towards themselves, or other fish. In fact, some of my smaller fish, Jade eyed Cichlid and severums, hold their own against the much larger Oscars.

You've got a big tank so they have got quite a lot of space to move around in. In many cases, it is actually a lack of space that increase aggression problems. Never tried it myself, but people talk about rearranging the tank. However, since he added the Oscars after the other fish, I can't see how this would work. In fact, I don't see how that would work anyway. If you've got an aggressive fish, he isn't going to be any different if you move rock to the other side of the tank (in my opinion)

It's difficult to know what to suggest. If you are having problems where fish are being injured, I think you should think about finding a new home for the Oscar. After all, it's better to boot out the bullies and the not punish the ones that are behaving themselves. I would imagine that your red tailed shark is quite settled, it would be a shame to move him just because you've happened to purchase a nasty little Oscar. If I was in your position, I would get rid of the Oscar, who wants aggressor fish anyway? No matter how we love our Oscars. There is no way I would put up with an aggressive fish that was injuring my other fish

Basscally it took your oscar a few weeks to settle into its new home and now its taking over re arrange your whole tank again so his surroundings look different this will give the shark and gourami sum time to heal and the oscar sum time to sulk or reflect on his bad behaviour if this happens again im afraid you gonna have to make a desision and decide to split the bunch up as you dont really want dead fish on yr hands,another suggestion could be to put a devider in yr tank temporaraly until the damaged fish heal,deviders are really cheap from yr LFS.

Welcome to OFL!

Man, that's a shame, what you decribed could have been a really, really nice setup.

What you said about the white spot makes me wonder a bit if it could indeed have to do with that. I'd sure think the ich makes fish feel unwell, so maybe there's a small chance the Oscar is just "letting it out" on the others. I'd rather think what the others said though, that it's just an aggressive Oscar. Whether you want to try that rearranging thing or not, you could try to create some hiding spots for the shark where the Oscar doesn't fit in, if that is possible. Was only thinking how they are more slender than Oscars and very fast as well. Hope it survives anyway. Wouldn't help with the gouramis though.
A divider is definitely a good thing to have handy anyway with cichlids, but in the long run there would probably be no other way than taking the aggressor out if it stays like that.

The same thing happen to me,I had a 55 gallon tank with 2 silver dollers.a huge goldfish,10 inch pleco,2 baby sharks,and then i put the oscar in.again it was 6 months and the oscar ate the sharks and beat the goldfish to near death,dident touch the silver dollers.Before i finnally got his new tank,put the oscar in the new tank and everything is fine now.oscar is still a little mad that he couldent finsh them all off but of well..so imn my experience oscar dident do well for long in a community tank.

I have two ideas that might help you:

1.) If you have another tank, put the bad oscar into it. Turn the heat up, and add some ich medicene, because you said he had some.

2.) Get some dividers and seperate the evil oscars from the rest(this may cause the other ones to get ich though

In my opinion there is only one to do when you get an agressive fish and thats remove it from the tank. Either take it back to the LFS or put it in its own tank. My friend had 4 oscars and one of the smaller ones used to go underneath the bigger ones and attack them. So we put the aggressive one in my tank and it did the same to my oscars. My friend took it back to a LFS. Oscars are generally peaceful but some are outright bullies and as OFL says who wants a bully

Many thanks everyone for all the replies. Nice to have some quality advice.
Well, I have another tank setup ready to move the oscar over if things get worse. We are having a good day today, he is quite chilled out and the white spot has almost gone which is really weird as I have not treated the tank yet. I have put Melafix in the tank 2 days running now to treat the other fishes wounds. Fingers crossed they all learn to live with each other. If not, I will move him out of there quickly.
Oh, when I got up this morning, he was SULKING!!! Hiding in the corner!! I could not beleive my eyes!! These fish amaze me every single day and I will be really gutted if I have to let one or both of them go.
Will keep you all updated and thanks once again! Stuart.