Danio behavior

A little background. I've had 7 danios in my 27gal tank for about a month and a half. I've just upgraded from a sponge filter to a HOB and I've noticed that my zebra danios have started to school where before they would be all over the place.

Anyone else had similar behaviour changes with a change in flow?

 

Schooling is a natural behaviour of danios, so you should be happy to see it.

The only good reason I could imagine the sudden schooling is a reason of energy conservation - much like marathon cyclists, individuals take the lead to bear the brunt of the drag, reducing energy requirements of the rest of the school members following them, increasing the foraging effectiveness of the school as a whole.

 

It could be that you have now changed the flow and the fist are unsure of the change. perhaps once they get used to it they will spread out again.

 

It is normal behaviour from danios to school. They love water flow in a tank and the sponge filter probably wasn't doing it for them Now that you have switched to a HOB they will start to show their natural behaviour