ahhh my tiger barbs are dieing one at a time

[peaf]

i had 6 of them but every 2 weeks anouther bites it im down to 1 now (not a single one is dead from all my other fish), i have only had them for about 7 month, they are probably the most agressive fish in my aquarium, water quality is top notch, can some one point me in the direction where im going wrong.

 

[Phantom Thief]

They're killing each other? Never heard of them killing each other, but then again, i don't know too much about barbs.

P.T.