Heat coconut oil in a large, nonstick skillet over medium-high heat. Once melted and hot, add the onions. Sauté for 1 minute, then add the carrots; cook for 2-3 minutes before adding the peppers and garlic. Sauté another 2-3 minutes.
Move the veggies to the sides of the skillet and add the red curry paste and peanut butter; sauté the curry for 2-3 minutes to intensify the flavor, moving around the pan. Then add the coconut milk and fish sauce. Bring to a gentle simmer, reducing heat to medium-low.
Add the mahi mahi, gently nestling the fish into the curry. Simmer for 10-12 minutes, until the fish is cooked through and the sauce thickens, stirring occasionally. Remove from heat and stir in the lime zest and basil leaves. Serve over rice, if desired.