feeder guppies how to have them survive?

[turtlefish]

I recently aquired 12 feeder guppies. They all looked fine when i got them. Now I think 6 died. They seem to be covered with a white sort of fuzz growth on them (dead ones only). All of the dead ones lose their greyish color and are white. Its so hard to find the dead ones, they lie in the substrate and remain hidden. I have a whisper filter and its a small 5 gallon tnak with clumps of anacharis. Whats wrong? How do you find dead fish? I know it is easier to put them in the turtle tank but i want to breed them. I am now afraid to put them in to feed my turtles because so many are dying. I seem to never have succcess with guppies!