Predator killing fish

Question:

Hi I have had my 7 goldfish/koi in a outdoor pond for nearly 3 yrs.. but 2 days ago i added this very large “sucker” fish and now i have lost 4 of my 7 goldfish AND the suckerfish!!! I don’t know WHY this happened? the goldfish were FINE.. could it be because I added the suckerfish? I greatly appreciate any input and/or info you may have THANK YOU Denise Von