This is my little red eared turtles image eating my pet gold fish which died of natural causes. Should I be worried? Is there anything I should do?
Firstly, you shouldn't keep fish and turtles together, the habitats are too different to be properly replicated in a home environment (you'll never have enough room and hiding places for them for example).
If you feel you must give feeder fish, these should be kept seperately until feeding, so you can make sure they are free from infection (as many feeder fish stock are badly maintained and could possibly be medicated, which wouldn't be good for your turtle).
As the turtle has already eaten, there's nothing you can do now other than keep an eye on your turtle, make sure it's waters clean and you keep on top of all tank maintenance.
While you can find sources which say this kind of feeding of feeder fish is fine, you can also find plenty which say it's a bad idea. My sources are largely from talking to people I trust on the subject however here are some links which I found useful:
Turtles and fish are always a bad mix. Goldfish are dirty and turtles are dirtier. Fish cannot survive in an environment high in organic waste. Fortunately, turtles have cast iron guts and can eat just about anything. Still, keep an eye out for both parasites and illness, just to be safe.