I'm leaning towards the Red Tailed Shark as being the culprit. Red Tailed Shark's are a little less agressive when in a
55g or more tank.
If you can, take it back to the LFS you got it from and see if you can get a credit for it. Explain that since it's addition you've lost one fish and it is stressing out the other occupants. Once the tank has regained some normality again and the Angelfish's fin has grown back some, then you can go back to the LFS and use your credit.
Incidently, with 6 Neon Tetras, 5 Zebra Danios and an unknown number of Angelfish, you may be overstocked or at least close to it. If so, and as a suggestion, you could use your LFS credit for some live plants, decor or even towards another tank
