ive got two oscars. ones a bit smaller than the other. they are 7 inch and 5 1/2 inches. the bigger one bullies the other occasionally. does anyone think that this will lead to the death of the smaller fish? it never has any marks or anything, just gets chased around, it will last for a couple of mins and then everything settles down. i was just wondering if stress would be an issue. they are in a 120 gallon long tank with an 8 inch common pleco.

Although it does happen occasionally, it's not a common occurrence where one Oscar kills another one, I've never ever had it happened to me anyway. However, if you think that one Oscar is taking too much of the bashing then you may have to separate them

Fish do not play, they don't have the brain capacity to have those kind of emotions.

they quit all the chasing and everything today, but now they shake their tails very vigorously when swimming around. is this some sort of territory thing?

def sounds like the other oscar is marking out its territory & that is sometimes the entire tank depending on the size of the tank. I had a couple of oscars in the same situation, you might even notice some digging of gravel taking place.

just keep an eye on things, as long as the little guy is not getting any damage then he should be fine.