I have 2 red Oscars, housed in a 105 gallon tank along with a small group of S.D's and a Plec. The Oscars have co-inhabited the tank for around 9 months very peacefully and have grown from around 2" to their present size of around 8".

Over the last couple of days the fish have suddenly developed a strong dislike for one another, starting with jaw locking and then degenerating into full on war. Unfortunately (and despite being almost identical in size) one of the fish has got the better of the other and is now relentlessly nailing it every time it ventures out from under the filter. I'm becoming fairly concerned for the well being of the 'victim' fish and it's in quite a sorry state now.

Has any one experienced similar, and is it likely that I am going to have to seperate them in order to avoid a fatality. Any help or advice would be welcome!

i went thru the same thing.

is when they start becoming sexually mature. puberty if you will. theres not much you can do except try to separating them till they calm down, if they even will.

unfortuantly i had to donate my battered and bruised oscar to the LFS. the other wouldnt have a scratch on it. now shes a big 11" female thats looking for a boyfriend. i often wonder if i had left it in there with her if he would have turned into a mate for her or if i just had two girls.

Welcome to the site. Actually, you are not alone in having Oscar's that get along nicely and then all of a sudden for no apparent reason start a mini war in the tank. However, when Oscars become sexually mature and start thinking about breeding they can start getting quite aggressive with each other, as does happen with many other creatures in the animal kingdom. So it's sometimes a case of trying to determine whether this is just the start of a breeding ritual, or full on aggression.

I think it's early days at the moment. I certainly wouldn't like to start suggesting that you split them up now. It may be best just to wait a week or two to see what happens. You may find that they calmed down and start getting on again. I think they are little bit young to breed just that the moment though, so it may be a case of them approaching sexual maturity and maybe hormones are starting to kick in. You very well may have two males which can cause problems sometimes.

Give them a little longer and see what happens. If this behavior just continues relentlessly and it is quite obvious that one of the Oscars is going to have a pretty miserable time if they are kept together in the same tank then you may have to split them up.

Thanks for the input, will see how things develop over the next week or so and keep you informed.

^ yeah, what they said
welcome to the site

LOL, They got you covered!! Welcome to the site Hun!

small scrape or few lost scales will be fine. just observed when feeding,
if one do not take foods, then he/she is truly hurt. but as long as each
one compete for their share... no need to worry. each is just muscling up
trying to find out who is the boss.

If things get real bad then put a divider in. Wait for the injured fish to heal and then put the aggressive fish in the other side. If they get on remove the divider after 2 or 3 days. If they still fight then rehome one fish. If you have to get rid of a fish then get rehome the aggressive one

Things are looking up, yesterday the 'victim' fish was allowed out from under the filter to an extent, today things are back to relative normality with just a small amount of gill flaring and shimmying but no out right aggression.

The fish are actually pre-occupied at the moment mopping up the eggs of my silver dollars which are spawning today! Both fish seem to be healing in rapid time, most of the damage seems to be fairly superficial.

Nice to hear that.
And now for the SD's spawning???
What did you do to get them that far?
Anything special? Floating plants?
Watertemperature?
just curious, my SD's don't seem to get it!

Don't have too many problems getting the S.D's to spawn. There are 3 in total, 2 girls and the 'silver stud'. My water temp is fairly high (around 28), basically because I live in Spain so that's more or less the ambient temperature in the apartment, don't have to use a heater at this time of year. They seem to respond to water changes, I usually do about 120 litres per week, the male develops 2 black spots on it's side and it's fins and belly colour up red. It then goes after either of the females, which ever one is ready to lay eggs. They chase each other around the tank and egg scatter in the gravel. No floating plants or anything like that though, and the water is actually quite hard, although I use neutral regulator to keep the ph down.

my males do develop the spots and fight eachother all the time. There not interessted in the females. could be water temp. Mine is 25oC.
Maybe i should play some Barry White songs lol.