My bristlenose suddenly got aggressive

[Ulan]

I'm somewhat surprised about my bristlenose lately. It was a very peaceful fish up to now, but this suddenly changed. He's started to hunt down the cories whenever he spots one. He's also attacking every shrimp he sees or the tetras if they come to near, but his hate of the cories seems to be burning deeply, whatever the reason is. Only the otos manage to make a stand, because they fight back.

On the plus side, the bristlenose got a bit more personal. When I'm there, he looks at me and seems to wait for his algae wafer instead of hiding in a hole. I just hope he doesn't get a tank terror and it's only a phase.

Is this mood change normal with bristlenoses when they get bigger? He still hasn't reached two inches, and I'm a bit afraid of the things to come.

 

[TropicalNorth]

Bristlenoses (especially the males) tend to get more aggressive as they get bigger. He is also more likely to take it out on the cories because they are bottom-dwellers and he thinks they are invading his territory. I had to move my male bristlenose from my 20g to my 50g because he was bashing up my cories. This was when he was around 2 inches. In the big tank he doesn't display any aggression.

Chances are you will have to get him a bigger tank or return him or you could return the fish he picks on. It's possible the aggression will stop but not likely, and even if he isn't harming the other fish they will become stressed.

 

[Loach43]

Yep, your BN is probably a territorial male. You MIGHT be able to thwart his rampaging by putting more hiding places in the tank.

 

[Ulan]

Bristlenoses (especially the males) tend to get more aggressive as they get bigger. He is also more likely to take it out on the cories because they are bottom-dwellers and he thinks they are invading his territory.

I'm not sure it's the fact that the cories are bottom-dwellers. The bristlenose seems to be fine with the otos and accepts them grazing right next to him. Even the smallest oto manages to push him off the algae wafer by ramming him head on into his side. The oto likes to hang out with the bristlenose, anyway. And the bigger oto is nobody he wants to get into trouble with .

Perhaps, it's the always erect black dorsal fin of the cories that the bristlenose sees as provocation?

Yep, your BN is probably a territorial male. You MIGHT be able to thwart his rampaging by putting more hiding places in the tank.

If the cories only knew how to hide. I think one can describe cory behavior as "inconsiderate". They never know when to keep out of the way, and they usually bulldoze through the tank without looking left or right or what they bump into. This means that the cories will be back ploughing through the substrate right next to the bristlenose half a minute after he chased them through the tank. Maybe, this just means they are not really afraid of him and I'm putting too much thought into this.

I'll keep an eye on the further development.