If they’re anything like the PA pelletheads, you need only take variations of 3 flies for the first 2-3 days:
Buggers in black, white and olive, with and without flash & beads.
Eggs in hot pink, fluorescent chartreuse, and bright orange.
Variations (softhackled, beadheaded, flashy, etc.) of a rather large (10-12), very buggy, brown generic nymph.
Start with the buggers. Cast out beyond cruising fish and strip it in as fast as you can manage. Fresh stocked fish are highly competitive and getting 4-5 fish racing to your fast bugger is not at all uncommon. After catching a few, they may get turned off. Switch to a slight variation…either flash or color. Lather, rinse, repeat.
Failing that, go to either of the other two, starting with the eggs, which are bright and easy to see. The fresh fish will go after these as well. Either drop it in just in front of the cruisers and let it drift down through the water as they pass, or cast well ahead of them, and twitch slightly when they get close…almost on top of it. Again, reaction/competitive/agressive strikes are the game.
Finally, that brown thing represents both the pellets that have been synonymous with food for that trout’s entire life, as well as a fair approximation of 95% of what his new diet will become. To fish as a pellet: throw a small handful of pea gravel or other tiny stones on water near, but not right above, cruising fish to simulate feeding time at the hatchery. Then toss in your fly and let settle slowly. To fish as a nymph, any type of cast & retrieve may work…try them all. 
Stocked fish, IMHO can be much more difficult to catch than wild ones, at least for the fly angler. They dont behave like normal fish and as a result, its hard to read & interpret their behavior, let alone turn that into success. After they’re in the water for about 5-7 days, they’ll be REALLY tough to catch for a week or so, especially if they see alot of pressure. They’ll approach all offerings with caution, and midges may be your best bet. After a week or two of that, they’ll behave much more like normal fish.
HTH
