Why are my danios so aggressive?

I had five Zebra Danios in my 10 gallon tank. After a year of being peaceful, two of them began bullying the lone Tiger Guppy. I moved the danios into a new 55 gallon tank based off some advice from here. The danios have been good in their new home.

I just introduced a group of 12 corydora trilineatus today. They are the first round of stocking. One danio already started bullying the corys! Luckily they are fast, but I was bummed to see this considering the poor guys are probably already pretty stressed. I also figured the aggression would have stopped since the danios have more space.


I was originally going to have 20 tetras in the tank, but figured I would do 20 zebra danios instead since I had to bring the five in from the other tank. My question is, is this a bad idea? I figured they would be less aggressive if I brought in a bigger school. Now I am worried that they will all be aggressive and go after the peaceful little corys. If I don’t bring in a larger school of danios, I’m wondering how to stock the tank around them…

 

Not sure. My danios always picked on new fish for a couple days then went back to picking on each other.

 

Not sure. My danios always picked on new fish for a couple days then went back to picking on each other.

Perhaps I will just wait and see what happens. My main concern now is getting a school of 20. Last thing I want is for them to be a gang of terror to the other fish.

 

Zebra Danios feed vigorously at all levels of the tank and no food will be left over for Corydoras or others to eat.

Consider keeping them alone, I found them very satisfying to keep this way.

20 is a good number as no fish will be singled out by the others as weaker fish will simply mix and disappear into the group. Even visibly weaker fish thrived in my group.

Relatively cool temperatures are good for them.

 

I had 6 danios in my 29 gallon,I decided to rehome them back to LFS.I found that they were chasing each other and some of my other stock constantly, not what I was looking for in my tank.Sticking with my current stock, tank in peaceful again.Maybe they needed more room for bigger school I could not provide.

 

Do you have a planted tank? Hiding spots for the Corydoras? Danios are a lot like out of sight=out of mind.