Fish and turtles are an iffy subject - no matter what you do, when you do it or how you do it; Fish = Food. Thats the low and simple, however there are things you can do to increase the chances of survival.
Watch the fish in the pet store: (a)Do they always float around near the top? (b)Do they appear lethargic? (c)How do they treat the other animals in their tanks (own species or not)? (d)When you suddenly appear what do the fish do (yes scare them)? (e)Obvious but: Does your fish's habitat match that of a turtle? (Depending on how you answer these questions you will be able to figure out your own fish.
-(A)If the fish is almost always near the top, their chances of survival are increasing limited. Think about it - most of the food we feed turtles float. They are breach predators (the come from below and under their food). Basically what you provided was a pellet with fins - and thats how your turtle will view it
-(B)If the fish are lethargic in the tanks at the pet store, they will be lethargic in your turtle tank - this one requires no explanation me thinks.
-(C)Are the fish agressive or schooled? Both have their advantages and disadvantages. Agressive fish are solitary, tend to have hiding holes for added defence and usually grow larger. All good, as they will defend their territory (even against turtles), and have hiding places a turtle will at the very least have difficulty getting into. And as they grow larger they will tend to stay proportionately larger than the turtles mouth - this slightly discourages predatory nature... but does not eliminate it. As for schooled fish, they will hide but they are more likely to rely on numbers to confuse the predators. They do not go out of their way to protect themselves as they use their numbers (think flock of birds vs hawks)
-(D)When you scare the fish you want to see one of two reactions - a go to ground and hide (fast obviously) or a scattering then fluttering of the school. Anything else is not good enough vs turtles.
-(E)This one is obvious - if they live in colder or warmer temps, or higher or lower pH, they will not be at their best in the tank. Stressed fish are dead fish.
Now you have purchased fish that you believe will survive (there is no guaranteed species - remember that) what can you do to your tank to help them survive.
1) Low decorations that they can hide in, and turtles cannot
2) Plants or other thin high decorations that the fish can use to get around - Fish are infinitely more agile than a turtle. You give them a place to dodge, the turtle will lose interest trying to move in a way it cannot.
3) Feed them food that sinks. Let them skim the bottom eating, and watching the turtles. You want them there anyways to clean up the turtles waste, may as well supplement the diet there as well. Keep em in their preferred habitat.
Again this is no guarantee, nor are there any but: In my tank I now have:
3 Rainbow sharks (tanks large enough that each have their own habitat)
2 Plecos (I had three, but the larges of the sharks killed the smallest of the plecos - another problem with a community tank)
17 Odessa Barbs (I purchased 5, and they breed after each 50% water change - not all make it, but each cycle increases the number that makes it)
1 Mystery Snail (came with a plant - beware invisible eggs >_>)
12 Ghost Shrimp - these ones I buy as "treats". Okay for turtles to eat, help clean up a bit and are DIRT cheap
If your careful and pay attention you can set up an environment that a fish will survive in - and if they start breeding than you know you have a solid community tank (happy fish breed)
I have also tried some cichalids and tetras - none survived. I want to try a catfish - but I am waiting until the rainbow sharks pass on in a few years... I don't think they would get along.
