I just moved a new juvenile red oscar into my tank that was previously only inhabited by a 7 month old common pleco and female betta. I just had to move the betta into her own 5 gallon and my red is starting to step on my plec's "toes".

He seems very intent on taking over the plec's cave. He keeps getting run out and then the plec will stick his head out and stare the oscar down for a bit. This is a pretty big deal because I usually never see the plec out during the day.

Is this just the normal felling out phase that will pass or should I monitor this situation?

Sorry this is so long... and thanks

Oscars can be bullies. I wouldn't class them as an aggressive cichlid but they do have their moments. Most Oscars I have ever encountered are very confident and if they want something, they don't normally let things stand in their way. Most fish don't bother them so they will rule the tank on a lot of occasions. I would monitor the situation, if the Oscar is harming any other fish then he may have to decide what you are going to do. However, if it's just a little bit of bullying that isn't too bad, I would just let them get on with it and see what happens. Pleco are nocturnal so isn't going to be confined to his cave all time because the Oscar will have to sleep sometime.

I guess this is just a battle of bullheaded fish. The plec is very much used to running the place and while I don't think the little oscar could do much damage to him I'm more worried about him getting stressed out.

I'll just watch the situation for a few days and hopefully they come to an understanding.

That's all you can do really. As long he's got a really good hiding place where he can get away from any other fish, I'm sure he won't get stressed. I am surprised the Oscar is bothering, they are normally hang on that surface waiting for grub

I wouldn't worry about it. My 16 inch common pleco used to get kicked out of his cave all the time by my cichlids, but now he's grown up, and kicks them out the caves. Common plecs are big fish, they get over two feet it good conditions. They are powerful, and armored, beware, not even the most vicious adult oscar is built to deal with these prehistoric armored catfish.

I dont think you oscar will bother your plec in the long term. He is just throwing his weight around. It is unusual for an oscar to bother a fish for a long while- they dont hold grudges. Just keep an eye on the the plec to make sure he isnt being injured