What should I feed my turtle, Guppies or Goldfish?

People say that goldfish are bad for your turtles because they have a chemical in them that may kill your turtle. Also they are fat too. People recommend feeding minnows and guppies but the problem for me is that my turtles are too slow to catch minnows. I haven't tried guppies though. I'm currently in the breeding process of guppies and I hope that guppies are as slow as goldfish but as healthy as minnows. I used to feed my turtles goldfish but now that i've learned that guppies MAY be an alternate, i can hope my turtles will eat them. But still can you answer my question if i can still feed my turtles goldfish or not. I only fed my turtles 10 goldfish a month (5 every other week on either saturday or sunday) and for the rest of the days i fed red leaf lettuce, and turtle pellets with some shrimp pellets.