Bring a large pot of lightly salted water to a boil. Place pasta in the pot, cook for 8 to 10 minutes, until al dente, and drain.
In a large bowl, whisk together the mayonnaise, milk, pickle juice, dill, salt, and pepper. Mix in tuna and onion. Toss with cooked pasta. Cover and refrigerate 1 to 2 hours before serving.
