Quick story....
I had an Oscar I got when it was about 2".
It shared a tank with a single sailfin pleco.
About a year into these two I noticed the pleco pretty much ran the show. It didn't bother me much and the fish both grew together until they were both about 6".
This all started in a 55. Then came the 210g. Some additional fish led to more fish and then more still until I had quite the community tank and all was well. As the years went by and the two original fish grew to around 12" the sailfin still ran the tank...then one day it all changed. The Oscar suddenly wanted everyone out and the sailfin was suddenly banished the the far back corner and was seldom seen.
What happened? Who knows. Maybe the Oscar finally "snapped"...maybe it reached a certain age and as a mature Oscar it started acting as such...whatever the reason, it became a constant battle keeping the Oscar from devastating the rest of my fish as well as the tank....and one would think in a moderately stocked 210 there would plenty of room for everyone....especially since all the peaceful coexistence before....
The point is, all we can do is try and match up our fish the best we can. I see my goldfish act aggressive sometimes...yeah, goldfish...
So who knows what causes the behavior sometimes encountered unexpectedly. ..
but usually the behavior we observe only can be dealt with in limited ways.
I eventually re-homed the Oscar to it's own 180 and the community once terrorized suddenly were happy and peaceful again.
