I don't think it has to do anything with the water...
Something like that happened to me too. I bought two serpa tetra's and they seemed fine...for a while. A few days later the lager tetra started to really get at the smaller one. It got so bad that i had to seperate the larger one from the smaller one by putting it in a seperate container out of the tank. After a while, my guppy started to attack the other fish i had.O_O
So i decided to put the smaller one back, and took the large tetra and the guppy into a breeding net, and left the rest of the nice ones inside the tank. I waited a few weeks and decided to let them free. By that time the small tetra grew as big as the other one and now they only fight ocassionally, and the guppy gets along with others...
anyway if you totally skipped over that, just get a breeding net for the violent ones...and hopefully they would nip at each other d:
That is. If they're small. Or get a bigger net if you have a bigger tank