how big is the tank? what kind of turtle? chances are you wont be able to put fish in there that are not intended as food. and 20 for 5? that's pretty steep for feeder fish. considering when i buy feeder fish (golds/minnows) i ask for a dollars worth and get 10-12 fish, you're getting ripped off.
I can understand your want for more out of your turtle tank, but pretty much what you put in there will get nipped or eaten. you'll have much better luck providing a better aquascaped tank- mebbe a paludarium project. that might satisfy the boredom for the time being.