Preheat the oven to 450 degrees F. Put the potatoes in a saucepan and cover with cold water by 1 inch; season with salt. Bring to a boil; reduce the heat to medium low and simmer until tender, 10 minutes. Drain.
Combine the buttermilk, mayonnaise and relish in a large bowl. Add the potatoes, sliced celery, scallions, 1/4 teaspoon salt, and pepper to taste; toss. Refrigerate until ready to serve.
Whisk the egg whites in a large bowl. Put the breadcrumbs in another bowl. Season the fish with the paprika, 1 teaspoon salt and a few grinds of pepper. Dip each piece in the egg whites, then dredge in the breadcrumbs, pressing to coat. Transfer to a rack set over a baking sheet.
Coat the fish on both sides with cooking spray. Bake until golden brown and just cooked through, about 12 minutes. Top the potato salad with celery leaves. Serve with the fish and lemon wedges.