That happened to my Juju too. All of a sudden he got very aggressive towards my swordtails. So I got him a female betta. Yes, burn me if you'd like. Most people would not recommend doing that.  But he was ok after that. He didn't even attack her as much as he did the swordtails. They lived happily together and even had eggs (without fry though.)
Maybe you can get some more plants or something to provide more hiding spaces. Do you have a full tank shot? We'd like to see your tank. 
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55g
7 Zebra Danios
4 Cories - Wasabi (m), Hobbes (f), 1 (?), 2 (?)
2 Otos - Otto (m), Sonja (f)
Divider
1 DP-Weebo (f)
6.6g
2 Cory fry - born 9/25/08
Mini Habitat
60 Oto Fry
14 Oto Fry in Eggs
5 Oto eggs |