Introduced a new girl and she's dying

[aduffdalt0729]

New betta momma here. We have a 55g, established 3 female betta family that all got into the 55g rhe same day as the new girl, and a pleco. She just didnt seem to be fitting in, and now today she is floating at the bottom of the tank with a clear, slimy bit on her back?
1. Is she dying/dead?
2. What did I do wrong?
3. I posted on reddit and apparently my sorority is not balanced and they will all die soon, but that I cant get any more until they die because 3 isn't enough, but it's enough to kill a fish

 

[Colin_T]

can you post a picture of the injured fish, make sure the image clearly shows the issue?

if you added a new Betta into a tank that already had some Bettas in, the fish that had been in the tank for longest will have considered the new fish as an intruder and bashed it.

move the sick fish into its own container/ tank and keep its water clean and see if it survives.