Agressive Danio?

Danio rerio (zebra fish) like to be in schools. In the wild these schools are very large. In a school a fish feels bigger, safer and usually does not feel the need to act with aggression. In an aquarium you should keep min 6 schooling fish so that they can stick together and establish heriarchy between the species; preventing lash outs at other fish in your tank. There should be more females than males as well so that a female does not become over burdaned with male attention. D. rerio are very hard to sex, however, i have never i had working knowledge of other speceis in the genus Danio.

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Evanleigh said:

Thank you. So I guess 3 isn't a big enough school even though there are 2 females to 1 male.
Maybe they are trying to sex. Both the females are very fat. I see you have just 4 Danios in your 20 gall. Is one more female going to change the way the male is acting?

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I do not think it is a mating thing. He is just trying to school with the slower fishes. I would try and add 4-6 more and let the Danios school together.

Yep, a common problem when you have too few Danios. As previously mentioned they need to be in larger groups to be happy. I try to keep at least 8 as a bare minimum.

Dennis

From my experience (other fishes)==No.

But Zebras, are zebras. Same fish, just different color.

As others have correctly said, danio (as all cyprinids and characins too) are shoaling fish by nature, and the larger the number the better. Six is often recommended as minimum, but I agree with whomever here said eight would be better. Danio are active fish, hence they swim more than say rasbora. So group size has to take into account tank size as well.

But, if you don't like them, I would try to re-home the three rather than adding more. Bear in mind, that adding say another 5 or 6 may not (= almost certainly will not) rectify the bully problem. This is something that occurs from the stress of not having a large enough group, and unfortunately the damage is permanent. A scientific study a year or so back, the first of its kind, looked at aggression in shoaling fish and discovered that with fewer than five (for the few species tested) aggression increased significantly. And this involves the species; mixed species is not the same thing.

Whatever you do, do something. A "bully" is going to cause stress to the other fish, and as this continues it worsens. This weakens a fish's immune system, making them susceptible to disease they would otherwise be able to fight off. It may also increase their aggression, so it just worsens.

Byron.