Why are my bangais fighting

[wasabi bean]

I have had a pair of bangais for around 9 months at this point and there has been a small amount of fin nipping from time to time but never harmful and never affected the small one and two weeks ago i moved them to a larger tank and have been fine then overnight the large one has destroyed the small ones fins and it is constantly hiding, what could have caused this?? You can see the damage below

 

[F i s h y]

My guess is that one or both has become sexually mature and they are both males. ive been breeding them for a couple years now and the timing matches up.

 

[wasabi bean]

My guess is that one or both has become sexually mature and they are both males. ive been breeding them for a couple years now and the timing matches up.

Well one did get a lot bigger than the other one (2x bigger) and they both started the same size, the other one has hardly grown

 

[wasabi bean]

For now i have put up a mirror to hopefully reduce aggression but i will have to keep a close eye on them