Today I introduced a new Oscar to my tank. My old Oscar Frank Lucas is decently huge(about 6 inches and fat) and my new Oscar Konstantine is relatively small about 2 inches and skinny. Frank Lucas is pissed and keeps chasing Konstantine and jabbing at her pretty rough. Every time they see each other Frank Lucas decides to jab her and Konstantine doesn't fight back and just cowards away. Should I be concerned?

I wouldn't be too concerned about the big Oscar jabbing your smaller one.BUT, do a tank change though for the better if you are worried because some bigger Oscars do pick on smaller ones as a terriotal thing and as a senior thing like this is my home get out type of tempermeant.SO for your smaller one for now I'd get a 10-20 gallon tank for it til it gets a lil bigger say around 5 to 6inches then put them in the tank together again and see what happens that's just my opinion on that situation,good luck

well never mind i just went out to the bar for 2 hours and i came back and he killed her... thanks any way thats 7 dollars i'll never get back

If you want to add another at a later date it is advised you try very hard to match size or a little larger.

haha i actually bought the largest one that Petco had... I'll just wait till they get a bigger batch.

in introducing new tankmate with oscar, alway remember the
golden rule: 1) feed first the host before intorducing the guest;
2) never introduce guest that is less than 1/2 the size of the host.

deviation from this rule will result in casualty.

that is probably what happened thank you for your response

How big is your tank? It is never a good idea to put two oscars together in anything less than a 70gal and that’s pushing it and they have to be raised together or you are looking at a big mess. Chances are Frank will try to kill anything smaller than him (and apparently he has proved this). If you are SURE that they are Male Female it might improve when she gets bigger than you worry about breeding witch can be a mess if you don’t know what you are doing.

On the other hand if you take your time to get to know the particular temperaments of your Oscars, you might find more often than not that Oscars are way less aggressive than their reputation suggests.

Huge size discrepancies can be a serious disadvantage for the smaller fish, and as a rule of thumb it is wise not to put two Oscars of way dissimilar size in the same tank. A DIY tank divider with easy to handle egg-crate might do the work, make sure you attach it firmly, Oscars are very strong fish.

When I brought home my second Tiger Oscar, it was 1.5" smaller than the other one. At first the big one went after the newly arrived big time so I had to split them. Once I manage to place them together properly, roles were inverted, and the smaller one dominated the big one. It turned out that the big guy was indeed a big gal, and the small fellow turned to be the man in the tank. They are always together, already paired-off, spawned, and attempted to raise their first fry.

I was very lucky to get, by pure chance, two young oscars that happened to be female and male, and that decided to pair-off; although they didn't have other options to choose from.

Pepe
Santo Domingo

That may have been the problem I had them in a 55

tank size really matters. but if you started
the oscars to be together from the start, it
will lessen the in-fighting.

i started raising oscars in a 18" x 18" x 24"
tank and they grow up at more than 12" long each
until i upgraded my tank. with good caring, you can
have two oscar in 55 gal tank but you have to give
extra time in maintenance.

but introducing guest at that size of oscar & tank,
you will be wasting your money and time. Territorial
supremacy will cost you that.

My one friend just started a 20 gallon a few weeks ago with an oscar that is about the same size as mine about 6inches.. i tried telling him how it was a bad idea but he didn't listen... just out of curiosity how bad is it to have a fish that size in a 20gallon?