This is not always the case, here is my female Bahia with a close up of her anal fin you can also see her vent.
Her Anal fin is not rounded. (excuse the torn fin lol)
Its funny my kelberi are so diferent I got them bothe at the same time
one is always so agrssive a lot bigger and a lot more defined in collor so i kind of thought that it were male and female.
But again thanks fore the lesson.