What kind of fish should I feed my turtle?

My turtle is too small now, but when he/she is bigger, I'm going to throw some fish into its tank for it to snack on. I was planning on small feeder goldfish you see in the petstore usually in one big tank. I recently, however, read in two places that feeder goldfish are bad for your turtle and both sites recommended rosie fish instead? Since, two sites have said goldfish are bad, I'm convinced I should re-consider. What are rosie fish and are they common? Like could I get them at an average Petco?