Zebra danio aggression

[IttyBittyFishy.]

Hi all,
I recently added 8 zebra danios to a 20 gallon planted tank (no tank mates for now). Anyways, I've never had this fish so I'm not very familiar with their behavior, but I've noticed a lot of aggression. It's constant chasing, and I noticed today that one of them has a nipped fin. Any idea as to why this might be?

pH 8, Ammonia/nitrite 0, nitrate super low, temp 74 F

 

[Autumnsky]

The pH is slightly high. Do you know if they were kept at the same pH at the fish store?

Also do you know if you have a higher number of males than females? If so, they will definitely chase each other a lot. They chase each other a lot all the time anyway, it is just how they roll. Very busy fish.

If you have too many males consider another group of females only to add to the group.