I have a 55 gallon tank with 1 male and 2 female yellow labidochromis
caeruleus. For the past few days, one of the females has been getting
harassed by the male and even the other female. She has been taking
refuge behind the heater. This morning, I noticed that her tail fin
was badly bitten and so I immediately removed her to a quarantine
tank, to which I added some melafix and stress coat to aid recovery.
She doesn't seem to be holding.

Was that the right thing to do? Can someone suggest some treatment for
her? And will it be safe to introduce her back into the main tank
after she's recovered? Will it be helpful to buy another female or two
to curb the aggression?

Thanks,
satyen