Well, Dolly, my molly, was a dalmation one and I only noticed it because she had this big white patch with no spots all of a sudden, I remember seeing something that treated it but it was a really funky name >_<. But, my other females are still good, she was the only one who got sick. Someone told me it was from stress, but she just gave birth when she started to look sick... how much more stressful could you be? lol
Also, my breeding pair bettas died after Dolly did, that same person told me that that disease is capable of affecting both mollies and bettas so that's probably why your betta died. They didn't show the same symptoms but I think it all resolved the same in the end. Must've been some fungal disease in my tank or something.