I have a 20g long set up with plenty of fake plants, a large cave, and some other covers (bridge, japanese latern, etc). I've had my male balloon molly for about 6 months now and my angelfish about the same. I'd heard mollies were fin nippy and balloons, while slightly less, could be too, but this takes the cake.
My balloon molly has recently gotten something up his butt about his angel tank mate. They've been fine all this time, there werent any tank changes when he became aggressive. My angel is pretty big, larger at 7"tall fin top to fin bottom (though skinnier) than the balloon. The balloon just one day woke up and decided he wanted a fight. Now he's chasing and nipping my angel all the time - except feeding. He singles him out and chases him.
I thought that maybe the lack of females (he had way back when, but she passed due to a bout of callamus worms) he was getting hormonal - I had seen him trying to get with anything in the tank that moved about a month ago, but the behavior stopped. So now I got him two females, but he barely bothers with them, still focuses on the angel. I notice he not only charges and nips, but he'll flare and show his fins too.
I'm unsure of what to do other than rehome the balloon since I dont want to do any more set ups. Anyone have any luck with mixing up the tank set up to destroy territories (though the balloon doesnt seem to have one spot - he doesnt even bother with the cave)? Seems that this is a balloon molly issue - sudden mad fish disease - where others have had balloons go off the deep end. Would a sparring partner (another male balloon) help?
My angel is pretty peaceful for the time being - not sure hes matured fully yet. I'm surprised he doesnt take a swing back at the molly.