Preheat the oven to 425°F with a rack in the center position.
Pat the tilapia dry with a paper towel. Season all over with the salt and paprika. Melt 4 tablespoons of the butter in a small skillet over medium heat. Stir in the garlic and cook until fragrant, about 1 minute. Remove the pan from heat and stir in the lemon zest.
Place the lemon slices in a single layer on the bottom of a 9x13 baking dish. Add the tilapia over top of the lemon, then pour over the garlic butter. Bake until the tilapia easily flakes with a fork, 10 to 12 minutes.
Meanwhile, melt the remaining 1½ tablespoons of butter in the same skillet over medium heat. Stir in the panko and cook until golden brown, about 4 minutes.
Divide the tilapia between plates and sprinkle with the toasted breadcrumbs, pepper, and parsley. Serve the lemon rounds on the side if desired.