Trouble in betta tank

[Alien Anna]

Hi Everyone,
I have a 12 US gallon tank with 8x black neon tetras, a pair of cherry barbs, a male betta and a snail.

Today when cleaning out the tank I noticed that one of my tetras has been attacked and his upper fin bitten off virtually to the quick. He is also covered in bite marks. One of two of the others also has bite marks and my male cherry has one eye. What on earth could be doing this?

The two culprits are Brucie, a male betta who has been living with them at least 6 months and one of the other black neons, who looks like a miniture piranha. Before seperating anybody, I'd like to know who is responsible. I've never heard of a betta turning rogue like this, but then again, I've never heard of a black neon turning cannibal either.

 

[FlareBettaGuy]

Bettas do not have 20/20 vision! Sure, they only attack their own kind, but they will also attack other fish which look like them, and they will often mistake other fish, such as guppies, for another male betta. You might want to put the betta in a seperate tank, at least one gallon. My betta is also living alone.

 

[Alien Anna]

Bettas do not have 20/20 vision! Sure, they only attack their own kind, but they will also attack other fish which look like them, and they will often mistake other fish, such as guppies, for another male betta. You might want to put the betta in a seperate tank, at least one gallon. My betta is also living alone.

Black neon tetras and cherry barbs look absolutely nothing like bettas!

 

[White Whale]

You left out one other suspect who could be the predator - the snail.

But, seriously, separate the betta first and see if there are anymore injuries that occur. If there aren't, then you know who is the guilty party.