10 points! WHAT I AM GOING TO DO WITH MY FISHY!?

I just bought a pair of guppys. The female was already pregnant. In the ten gallon tank already was two dyed white skirt tetras, and three black phantom tetra's. I put the guppies in. And the white skirts started nipping at them and bullying them. I caught the white skirt tetras. And they are in a seperate bowl. What should I do with them. Possibly what is a humane way to kill them.