Heat broiler. Arrange pineapple rounds on a rimmed baking sheet in pairs, overlapping slightly, to form planks. Lay a fillet on each pineapple plank and season with salt and pepper. Broil until pineapple is browned at edges and fish is opaque throughout, 5 to 7 minutes.
Meanwhile, cook couscous according to package instructions. Add lime zest and fluff with a fork. Stir together lime juice, oil, agave nectar, and jalapeno; season with salt and pepper. Drizzle fish and pineapple with lime dressing and serve with couscous.