Heat oil and fry tacos shells until a shade darker, about 1 minute.
Transfer to paper towels to drain and season with salt. (WE DO NOT FRY SHELLS)
Whisk together lime juice, 3 T. oil, and salt and pepper to taste for dressing.
Season fish with salt and pepper.
Heat remaining oil in a large nonstick skillet over moderately high heat until hot but not smoking, then saute scallions, stirring, 1 minute.
Add fish and saute, stirring occasionally, until just cooked through, about 3 minutes.
Remove skillet from heat.
Toss lettuce with dressing and salt and pepper to taste.
Fill Tacos with lettuce, fish mixture, and salsa