Help! Molly fish being bullied

TLDR: My molly fish is bullied by other mollys even if I swap out who is bullying. What do I do?

Here is my setup I have a 20 g tank, a 5 g snail tank, a 150g plus pond, and a 10g tank I use 9 months of the year (I am a teacher and have it in my classroom).

I have 6 female mollys in a species only 20g tank. One fish (Tecky) is bullied by several of the other fish. I moved one of the main bullying fish but then the other fish started bullying Tecky too. She has had fin injuries and now has her second time of a fungal infection from the damage and stress. I plan on putting Tecky and one or two of the nice fish in the 10-gallon during the school year. Currently, I put Tecky alone in the 5-gallon snail tank while I treat her and she heals. After that, I'm not sure what to do. My options are:

1. Keep her in the 5g tank solo if she seems healthier and less stressed. or
2. Put her back in the 20g tank with the other mollys and maybe she is OK, perhaps she gets bullied again and stressed.

Either way, it will likely only be over the summer. I prefer to have fewer tanks than more so I am not open to using the 10-gallon tank at home.

 

Is she smaller than the others? Or perhaps pregnant when the others aren't? I have seen female mollies bully the males to death, but not each other.

 

I'd leave her in the 5 gallon tank. I wouldn't recommend keeping a molly in a tank that small long-term, but it's only a few more weeks until school starts when you can move her to the 10 gallon tank.

 

No she’s the same size and she’s not pregnant. Some of them came to me pregnant but they’ve all given birth (at least once lol) and it’s been months since the last birth. I took the fry to my local pet store and they accepted them.

 

Maybe there's something else going on... Maybe a picture of the bullied one?

 

Maybe there's something else going on... Maybe a picture of the bullied one?

I think maybe she is just weaker. One pic is her in the communal tank before she got sick. Even before she has been sick the orange one and one of the smaller ones would nip at her. All of the fish have been sexed as female so I don't think it is mating behavior either. (also the little dot on her in the communal pic is actually on the tank glass, not on her)

 

Well, the more dominant ones do tend to pick on weaker and sicker ones. The white splotch on her shoulder in pic#2, is that from the picking? It looks like an injury but also like fungus, mucus or columnaris.