Hi Welcome to Fish Lore

I'm sorry about your Angel.
What are your readings for ammonia,
nitrite and
nitrate?
It's possible he's getting nipped. Unfortunatly, you have some incompatible fish.
Angels can sometimes be aggressive with one another in your size tank.
Imo, I'd blame the bettas for his fin. Most bettas don't make very good community fish. They can get very aggressive or be picked on.
Males and females shouldn't be kept together. Females will sometimes tolerate each other, but usually in much larger tanks and if they're sisters who grew up together.
My suggestion is to separate the bettas in 5g tanks of their own.
Here's our
Betta Care Guide
I hate to be so blunt, but you really should separate them before some one is more seriously hurt or killed.
After you get the fish sorted out, keeping the Angel's water pristine to prevent infection along with some added stress coat should help him heal. You can also add a few drops of Garlic guard or garlic juice and Fish Protector.
All of your fish are amazing, just not together.
Best of luck.
