i think it always diffecult when you have more then one betta in the tank. Once one gtet sick then give it to others. And i am personally think the more water changes less chance to get sick. So if you cycle the tank always do 50% water changes with regular vacuuming and filter media swish/wash. If you don't cycle do 50% and 100% water weekly. It the best. Some people recommending 25 % water changes which i think will be a problem sooner or later. I do a lot of water changes and my bettas don't get sick.
I am glad you was able to save one girl! Do you have males?