Help ID this. Zebra danio with red tail

[sobersteve323]

Need help with an ID. I can't tell if this is an injury or something else.

Noticed one of my zebra danios hovering near the surface last night with clasped tail fins. Today his tail section appears to be red on both sides. It doesn't look like fin rot but I'm not sure. I did see some of the other danios chasing it around yesterday afternoon but I didn't think anything of it until seeing it today. It's also not interested in eating today even though it ate last night.

All others in the tank don't have any signs of the same thing.

Water params:
pH ~7.5
ammo/nitrite/nitrate 0/0/10

This is the best picture I could get. Any help is much appreciated!

Edit: Added water parameters

 

[sobersteve323]

I figured it out. I caught one or two of the other danios nipping at it. I had no idea they could end up aggressive even with a school of 8! Looks like I will be returning them since I read they will even pick on other top level species if there is an overly aggressive one

 

[sobersteve323]

Update. This morning the redness was gone. But when I got home this evening it looked like most of the tail was gone and there was new redness. Will treating with melafix help repair the fins even if its been nipped almost to nothing?

 

[Talvari]

I could be wrong, but I've never seen a zebra danio's fin by nipped that far down by other fish, particularly other danios. The redness at the base is a concern... Id do some anti-bacterial medication and try to separate that fish from everyone else for awhile to see if he gets better. If it was fin rot, the tail will usually not grow back once the infection has reached the base of the tail.

 

[sobersteve323]

When the picture was taken the red part was the base of the tail and only part of the tips looked torn. The pic wasn't very good but it was clasped and still almost all there. Unfortunately he passed away last night though in quarantine.

As for the nipping I'm still sure that was the cause because they were still picking on him when I was setting up the QT. Pretty sad watching him constantly harassed.