Danio bullied by other danio fish

Hi all.
I had 6 zebra danios in a 30 gallon for about 2 months. One died on Thursday and now one of the other danio is being chased constantly by the other 4. He’s moving his mouth a lot and gets chased everywhere he goes in the tank.
anyone have any idea why? I tested the water with Api and all good, zero ammonia and nitrites. Ph has been around 7.5 since tank setup.
I have seen fish chase each other before, but not a group chase one fish

 

Welcome to Fishlore.

Some school fish get aggressive if there is a low fish count in the school. I would up the count to 15.

When I kept them in a 30 gallon the school was 20. Never had issues with aggression.

 

Welcome to Fishlore.

Some school fish get aggressive if there is a low fish count in the school. I would up the count to 15.

When I kept them in a 30 gallon the school was 20. Never had issues with aggression.

Thanks Thunder, I was thinking about adding a few more. Think I will do it soon to hopefully calm them down

 

Best wishes for peace in your aquarium.

 

Probably a female chased by males wanting to mate.