It's Mr. Swordtail.
The only thing I can think of is that he fucked them to death. I watched it with my own two eyes tonight. He just kept stabbing and stabbing, til Mrs. Swordtail just went to the bottom and stayed there. She's still there, gasping her poor little mouth. Maybe his horrendous aim went into her gill or something.
But I don't think she's gonna last the night.
Now to decide - keep him, or euthanize him out of punishment, and go with fish less sex crazy than swordtails. Or, just not get any men. Hmmm.
Life will get better. Yes, it will.