STRANGE and violent Zebra Danio behavior - Tropical Fish Keeping - Aquarium fish care and resources
 
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post #1 of 6 Old 01-16-2011, 10:53 PM Thread Starter
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STRANGE and violent Zebra Danio behavior

I have a school of 14 Danios - 2 Zebras and 12 Glow-fish - in a 4 foot long, 33 gallon tank and they usually just school around together peacefully.

This morning, before the timer turned on the lights, I noticed a wild and crazy scene. The males were chasing the females in packs and literally "attacking" them one after another. A group of 3 or 4 males would single out a female and "drive" her all over the tank at break-neck speeds until they would eventually pin her to the gravel and cluster around her. It looked as though they were trying to kill her dead. Eventually she would escape and the "team" of males would either chase her down again, or single out another female to make miserable for a while before moving on to yet another female victim. The females were terrified and this was obviously not a mating ritual - at least on the part of the hapless females. Though this happened to some degree to almost every female in the tank, one in particular withstood the brunt of it. It was disturbing to watch.

Shortly after my turning on the lights manually, it all ended and peace returned, but the females were obviously traumatized for a while.

What exactly was this strange behavior? Has anyone else ever seen this kind of activity take place with Zebra Danios? I hope I never see it again.

38 years in the hobby and counting
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post #2 of 6 Old 01-20-2011, 05:52 PM
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I used to keep danios with my Hypan. Contradens Pleco and they were crazy a lot of the time. One or two males would have every other danio scared behind a plant or into the rocks or whatever. Since they were obviously stressing the pleco out I moved them to my coldwater tank with my feeder crayfish. Since the move I have yet to see any aggression at all. They school very tightly without any problems. The tank is pretty low temp, around 65 degrees so if your tank is just danios you might want to consider turning the temp down.
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post #3 of 6 Old 01-23-2011, 11:12 AM Thread Starter
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Originally Posted by lorax84 View Post
I used to keep danios with my Hypan. Contradens Pleco and they were crazy a lot of the time. One or two males would have every other danio scared behind a plant or into the rocks or whatever. Since they were obviously stressing the pleco out I moved them to my coldwater tank with my feeder crayfish. Since the move I have yet to see any aggression at all. They school very tightly without any problems. The tank is pretty low temp, around 65 degrees so if your tank is just danios you might want to consider turning the temp down.
I have never kept them till this year. Though I have used them on many occasions as starter fish to cycle new tanks, they weren't on my list of favorite fish, so I would return them after breaking-in the tank and buy something I wanted. This time I decided to keep some of the colored Glo-fish variety and just noticed all this strange behavior for the first time. I have been watching and now know this behavior takes place every morning at dawn, after a little light enters through the windows and before the light timers kick in. I guess this is just something they do. Surely it must be some kind of libido/dominance thing with healthy males and normal enough I guess.

Turning the temp down much isn't an option in this community tank. Guess I'll just have to let the boys have fun.

38 years in the hobby and counting

Last edited by RCinAL; 01-23-2011 at 11:18 AM.
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post #4 of 6 Old 08-03-2011, 09:58 AM
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I am curious to know as well what might be the cause of this...
I bought 3 zebras less than a month ago, and the day after i got them i caught two bigger ones (i think they were males) beating up my platies by pushing them against the walls of the aquarium and then one went after one of my coreys, in which case it took of the top portion of the tail fin, i turned the light off and it seemed to help.

about a week ago i noticed my snail covering something with slime and i saw the danios ripping at it, after about 15 minutes trying to figure out what it was i realized that one of my zebras was gone.... they were eating what was left of it..

and of course just a couple days ago my aunt brought some neon tetras to my tank since her killifish were beating them up. i guess she brought 4 (i wasnt home at the time and have my dad taking care of the tank for a few days) i came home for a couple hours and was really excited to see the new fish, but when i looked they were gone, there was no evidence of any new fish in the tank....

i am pretty sure the zebras were the main culprits on that...im not sure what to do, i am scared to bring any more fish into the tank if they are just going to get eaten and i am told danios are not violent..so what could be the cause to this?
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post #5 of 6 Old 08-03-2011, 11:48 AM
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You were witnessing normal spawning behaviour. Fish generally spawn in the very early morning, just after dawn in the wild. And Cyprinids and characins (your Danio are Cyprinids) spawn by having the males actively pursue one or more females, attempting to drive them into plant thickets where the eggs will be released and fertilized, in the method known as "scattering." The sticky eggs fall among fine-leaf plants and hopefully survive predation.

This behaviour is stronger in some species than others. Danio, being very active swimmers, are more active drivers when spawning. This is why it is often suggested that there be more females than males. With more males, the females will be pursued relentlessly and often to the point of death. In an aquarium they have no escape, unlike the wild where they can swim away and "hide" among plants, rocks, wood, etc. And there are enough of them that the males pursue other females. In the confines of an aquarium, more females is preferred.

I recently witnessed this behaviour with my Emperor Tetra, the males of which are also very aggressive in spawning.

This is another reason why tank sizes are given for our fish in the profiles; some species need more space than others, even just for spawning.

Byron.

Byron Hosking, BMus, MA
Vancouver, BC, Canada

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian. [unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]
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post #6 of 6 Old 08-11-2011, 01:19 PM
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Seems strange... I have 4 Zebra Danios (I know that's not ideal but I'd be overstocked otherwise as I have 4 Harlequin rasbora as well) and my LFS told me that they're 2 male and 2 female but they play very happily...
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