Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This Skwala nymph still has a couple months left to go before hatching, but it's still a good representative of its species, which was extremely abundant in my sample for a stonefly of this size. It's obvious why the Yakima is known for its Skwala hatch.
Let's just say you were right. Based on the cast nymphal shuck, your specimen appears to be Dannella lita (previously classified as Timpanoga lita) or Dannella simplex (previously classified as Timpanoga simplex).
Were these photos taken in North Carolina? Was the emergence event, which under normal circumstances would be in June or July, perhaps artificially accelerated as a result of the water temperature in your aquarium?