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Hi there

We have 2 Oscar fish, 1 Red Oscar and 1 Tiger Oscar. They have grown up together for the last 9 months. The Red Oscar is about 8 inches long and the Tiger Oscar about 10 inches long. Until the last few days the little one has been the dominant one (oddly), but last night we found them having a proper rumble (the usual clamping of mouths but more aggressive this time)and have just come home from work to find them both with scales missing and the little one playing dead (which I have heard they do to avoid predators or allow danger to pass). The big one keeps harassing him and I'm worried that tomorrow we may have a casualty. We have heard mixed things - some say they will live together peacefully (and so far they have, we are convinced they love each other) and others say it won't last and they will start fighting until one is dead. Tonight we have resorted to getting a tank divider but we are concerned that might be the wrong thing to do if its just a phase. They have fought in the past but it seemed always playful, whereas now it seems serious. It may sound daft but they really are our pride and joy, they are so intelligent and dare I say, usually affectionate - so does anyone have any advice or experience of this? Many thanks in advance, Elle & Jamie

Hi elle&jamie welcome to fp! well i dont know too very much about oscars but i do know they get big and can be pretty aggressive. for you to have two of them in the same tank youd have to have a pretty big tank. How big is the tank you have now? it may just be a matter of getting a bigger tank. hope the divider doesthe trick for now!


The true test of character is not how much we know how to do, but how we behave when we dont know what to do.

Have you had them accurately sexed? Males kill males and females kill females, they only really coexist peacefully as opposite sex pairs, unless tanks are huge, literally in the 300 gallon plus leagues. Oscars are one of the most persistantly territorial cichlids you can buy, if things are getting to the point that one is stressed to immobility you should get cracking before something dies. You should really not have let things get that far. They will kill each other. Tank seperator should be in already, dont leave them to attack each other. Even if damage looks superficial, liplock fights can be exhausting and can cause death.

Thankyou for your replies!
with regards to having them accurately sexed, do you have any advice on how we can do this? We heard it has something to do with the ridge on the forehead. In regards to the tank question, have a basic 180 litre aquarium and were planning to get a larger size tank in January. We thought the smaller fish seemed exhausted and that was why he was being submissive. They are normally SO friendly towards one another, they swim together, play together, even stalk the Pleco together - but we were worried as they grew bigger this might happen.

yes you will definitly need a much bigger tank size soon. how big were you planning the new one to be? they still have a bit more growing to do. so make sure to plan for that as well!

The true test of character is not how much we know how to do, but how we behave when we dont know what to do.

Sexing an oscar isnt easy, an experienced eye will notice the subtle differences in build, and it really does take an experienced eye, no description will really help you, but TBH most of it is behavioural, and I think the oscars are telling you theyre probably the same sex. Once they reach sexual maturity and are in good health, smae sexes will start fighting, once mature they dont do "friends". Chances are, they will need to be permanently seperated. They can be so territorial, (and often hold territories of 8 cubic feet or more!) that its probably better not to assume a larger tank will fix the problem. A tank each would be better so that the hormones they exude into the water doesnt make them hyperaggressive even if the cant physically touch one another.

Might even find that you have to move the plec, oscars can be so aggressive about having any fish near a nestsite, whether they have a partner or not, that the plec could be killed in an effort to make his tank (his territory!) look more attractive to the inevitable partner he.she thinks will inevitably happen along.

Most oscars in the hobby, once mature, arent kept with other species precisely for this reason. They are a classic species tank fish.

If you post a picture I can have a crack at sexing them, but TBH a lot of it is guesswork, and behaviour is the biggest indication anyway. Im assumming you have fish that are the same sex, but whether thats male or female remains to be seen. Most oscars sold are males though. Breeders tend to keep the females back.

Just want to add to what's already been said.
In cramped quarters (and even if you have 55g per oscar which is the recommended minimum sometimes) 2 oscars (unless a COMPATIBLE breeding pair) will fight for territory and this usually will result in death. Having a male and female doesn't guaranty their compatability, which also can result in death.

Oscars are usually sexed by venting. Not sure how to tackle that myself. Most male cichlids have a longer top and anal fin that comes more to a point where females would be more rounded, and have a fuller belly making them appear a bit more 'squat' and not quite as long in the body. Not 100% though they can be very deceiving.

"We heard it has something to do with the ridge on the forehead." - You're referring to a Nuchal Hump which is not common on an oscar.



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