Some fish food can be synonymous with turtle food. We use freeze dried brine shrimp and the turtles LOVE them. They come in little 1cm cubes, and they break away into little shrimp in water - I haven't tried it with Matilda yet (I have her on a strict calcium diet because she's still in rescue mode), but Daryl thinks they are the bees knees.
When we were hosting feeder guppies in Daryl's tank, we would feed them fish flakes while he was eating his meal outside the tank in a separate bowl. When we'd put him back in, the little bugger would scope his entire tank for the fish flakes for the guppies. He's a champ.
Here's what we have:

I wouldn't be concerned about using fish food. I wouldn't use it all the time, but they have a lot of the same ingredients as the turtle food. Once in a while is probably ok.