At my college, we're only allowed to have one tank per room, with a 10 gallon maximum. My roommate and I, being avid fish lovers, bought a 10 gallon, dividers, and fish. I originally came to college with 4 male guppies, one of which passed away about a week or two after move-in. At the beginning of this month, my roommate and I got the 10 gallon and set it up, each with a betta. We divided the 10 gallon three ways, giving each betta 3 gallons and the 3 remaining guppies had a 4 gallon section.
My betta, a DSHMPK named Poseidon, is really aggressive towards my roommate's betta. Every time he sees him through the divider, he flares and wiggles. But whenever he saw my guppies, he would be completely calm and didn't care at all.
Today, I cleaned the tank for the first time and cupped both bettas. I moved Poseidon's cup to the guppies' side to float while I took out his decorations to clean, and he was still completely calm. They ignored him, and he ignored them. So as a test, I moved Poseidon to my roommate's portion of the tank (still in his cup) and he flared and went crazy. When I moved him back to the guppies' side, he calmed down. So, I let him sit for a while, while I cleaned his decorations. Then, I released him into the guppies' side. They got along great! No nipping, no chasing, no flaring on anyone's part. I sat and watched them for a good 30 minutes before I took everyone out to do a water change. I rearranged the tank and took out a divider, and now Poseidon is sharing a part of the tank with the guppies. They still seem to be getting along great, and it's been hours now. No bite marks, tears, or anything. They seem happy. Poseidon isn't being territorial towards them, and they aren't being annoying. :) I'm very happy, as this gives everyone more swimming space. My roommate's betta now has about 4 gallons of space while the others have about 6. I baffled the filter by stuffing some aquarium sponge inside it, and I baffled it again with a bit of a water bottle. The surface isn't disturbed, and no one's being pushed around.
Needless to say, I'm a very happy fish mama right now. :) I'll have pictures soon enough. My camera battery died.