Help, My Danio Has A Broken Tail!

So I have been in the cycling process in my 20 gal. long using TSS+ and my two weeks just ended. I did a water change and tested and my levels were great! So I happily drove to the LFS to go get 2 more Danios, as I had only had 3 in the tank up to this point, to round off the school. The 2 I bought yesterday were noticeably larger than the ones already at home in the tank, but had the same markings and such so I don't think I got a different species or anything. But everything went good, I accumulated them and they have seem really healthy. But I awoke this morning to see one of my original Danios had its tail broken It happened during the night so I'm not sure how but I really want to help him out

I can't upload an as it keeps telling me the files are too big

 

Try resizing the photos.

 

I'm also concerned about that string hanging from him, again all of this has happened overnight. Could that be a parasite?

 

When I came back from church he was tail up on the top of the tank. Not dead, but he had little to no swimming control and was breathing very fast. When I picked him up in the net he still struggled and squirmed but he would've had a very very low quality of life. His tail fin had fully fallen by the time I got home. So I made the tough decision and helped him pass. I'm down to four Danios and am planning on running to the lfs to bump the school to 5 again. But it still is a very sad moment. If anyone can still tell whether or not that's a parasite so I can start treating the tank that would be greatly appreciated

 

Bumping this for you.
Sorry for the trouble that you are having. I can't help you with your Zebra, I can only speculate. It's possible that it was bullied by the other Zebras, or just in bad shape from the LFS. I would suggest that you don't add anymore fish for a few weeks to make sure that the other Zebras don't come down with the same thing.