Soak the haddock fillets in lemon juice. Let stand for about 10 minutes, turning fillets halfway through.
Melt butter in a 9″ x 13″ baking pan. Drain fillets and place in pan. Season with garlic powder, pepper and oregano.
Bake in preheated oven for 10 minutes; remove from oven and turn fillets. Bake for another 10 minutes or until fish flakes easily with fork.
While the fish is baking, dice the tomatoes, chop the cilantro and finely mince the onion. Combine in a large bowl and drizzle with lime juice. Mix well.
Peel and slice the avocados. An easy way to peel an avocado: with a sharp knife, slice the avocado in half and twist. Remove the pit (it can either be scooped out with a spoon or take a sharp knife and gently tap the seed with the blade of the knife so that it wedges into the seed. Pull the seed out with the knife.) Slice the avocado and peel off the skin.
Remove the fish from the oven and shred with two forks.
Place 2 to 3 tablespoons of fish into each taco shell; top with 3 tablespoons tomato mix and fresh avocados.