Its not always the Male who is the aggressive one. In this case the female is much more aggressive. Ive been at this betta splenden fish game for quite some time and believe me i observe and regulate if needed, this is my 2nd pair ive had in cohabitation with out a problem.
I have had both of these little guys for a while know and I have observed both of them in my different environments. Gillian, the male, has been placed with many different types of fish, and even shrimp, with never an aggressive spurt. He sure does flair a lot though, but only at his own reflection. That makes me think that maybe he is only very aggressive toward males, so far this hypothesis is true.
Sun on the other hand has lived in my 10 gallon sorority with many other females prob 10 different ones through her time and has lived to tell without any fin damage at all. She has been roommates with Gillian for a good 2-3 weeks now with still no chasing involved. They always swim up next to each other whenever i feed and they never flair at each other. I put my other female Fluffy in to see how she would fair and Gillian my male had nothing to do with her, but sun was VERY aggressive and i had to remove her in just one day. She was getting chased around like nobodies business.
I have always been skeptical about all this online bull jive i have read about females and males cohabiting being certain death. I've always heard it from everyone on this site and others, and ive even preached it. But then i though you know what, i might as well try it and observe closely so i can really know that if what I'm advising against.
So far I have had two pairs of male/females living together with ZERO problems, ZERO aggression, and ZERO tails missing. From what i have observed it is really a personality thing. We all know our fish all have HUGE personalities, and honestly they are a bit like we are. Some times we just dont like some one for no reason, i believe its the same for these fish. Some get along great and some just hate each other.
I know some of your could be concerned i am treating my fish poorly, but in all honesty I am not and if there were any signs of it I would removed either immediately. I am not saying I advise anyone else to try this. If you do i would make sure you have almost the same stipulations as a sorority tank. Lots of hiding spots, dense planting, and lots of time to watch and regulate. Make sure you get to know your fish for a few months, try to place a molly in the his/her tank, see how they react, try an oto, try some ghost shrimp, try some snails. See how they react to each and gauge it! Put one female or male in a cup and placed in another tank with the other opposite sex betta that you wish to cohabitat with and see how they react to that over a period of a day or so. Make sure both fish are well fed and put them both in cups for a time and release them both into a tank at the same time so that you dont put one into a tank that has been "claimed" by their territorial nature.
Sorry for this obscenely long post, i just really thought it was time to inform all you misinformed people who have simply read things on the internet and automatically declared them truths. Dont get me wrong, i would never put two males together but some times even two females are far to aggressive to live with each other. Its just like finding the right temperament of dogs to live together in a small place, some just dont work and some work out way to well, such as Gillian and Sun.
If anyone has any questions please do ask, and PLEASE do not attempt this without reading all of the above and really considering everything i mentioned. I am sure i have left some points out but its early. Again please dont be offended by this or anything. I know what im doing, I have kept bettas for a quite some time. I have taken plenty of time to research and till my subjects prove me wrong, i am sticking to this. Ive kept two pairs together with ZERO problems so im 2 for 2 thus far.
Have a lovely day everyone! Hope you enjoyed my writings, though very extensive....