Late On-Set Zebra Danio Aggression?

[AquaticNovice]

I've had 4 Zebra Danios in my tank for about 8 months, ever since they were added as the first inhabitants to my 29 gallon bow front.

They have typically been quite energetic and prone to chasing each other around, but never to any harm. Last night two started aggressively pursuing another. This morning I found the one that was being chased is missing a big chunk of the tail fins (fortunately not into the base).

The chasing continued this morning, but to a lesser degree. This evening I plan to get my quarantine tank set-up and separate the harassed one out for a while.

What would cause this new aggressive behavior to emerge?


It's possible the one being chased was female, and the other three are male. Are Danios prone to aggressive spawning behavior?

I much appreciate the help.

 

[msquared]

I can add that I had four ZD as the first fish in my tank. They were fine, and also didn't harrass the four Peppered Cories I added soon after. After a month or so, one of them started become dominant and would sometimes chase the other three around. Then one night one mysteriously died, after having been just fine prior to when I found him. Then I added four male guppies, and again there were no obvious problems. The bully ZD still just chased his own kind. Then one morning I found both remaining non-bully ZDs dead. All three dead ZDs had fins intact, and no obvious trauma, so I can't prove the bully ZD did it. But I have my suspicions. Now all alone among a tankful of other fish, the bully ZD started chasing the guppies. I was prepared to add more ZDs, hoping that this would stop his agression to the guppies (and not yet fully realizing how bad he had gotten). But overnight I found all four guppies with shredded fins, and he had them corralled in a corner, hiding behind the HOB filter. That was it - he had to come out. So my experience is that yes, ZDs can become aggressive over time.

 

[jaysen]

i have a bunch of regular and long fin zd's, and my long fin zebras do show some nippage. i think the key is to have many so that a single one wont get harassed too much. i have 8 in a fairly large aquarium. i do notice some have missing scales as well but never any deaths. they breed like crazy though so maybe they get out a lot of their aggression that way.

 

[jm1212]

you may be noticing it more because you only havr 4 of them. try adding few more, and that should help distribute aggression among them better.

 

[carlyleigh]

I have a friend who had 4 pearl danios in his tank..... Similar story had them for around 8 - 10 months then they started killing the guppies.... It was carnage not a pleasant site... He then took them out.....

I myself have 4 pearls but due to the fact that I have larger fish have not had those issues.... Pearl Danios? or ID Sharks and Spotted Pictus who do you think would come off better....LOL