Could be Herichthys pantostictus if it's trying to kill all your fish, they tend to be a bit aggressive.
They come from the Panuco river system in Mexico, and are sometimes called "blue labridens", as they are part of the labridens complex.
And can grow into quite amazing specimens.
Here's an @ 10" adult male I had.

I have 10 juvies at the moment ranging from about 2"-4".
I believe I paid $4 each, but on average they are approx $8-$12.
Breeding color is quite dramatic, very much like their close cousin Herichthys bartoni with light color above the lateral line, and dark underbelly. below is bartoni