Marinate the fish: Place the pieces of fish in a large mixing bowl and pour the chermoula over them. Toss these together until every piece of fish is coated in the chermoula sauce. Let this rest and marinate for at least 30 minutes or up to 2 hours.
Assemble the fish tagine: Start by pouring enough olive oil into the tagine until it covers the bottom. This should be roughly ¼ to ⅓ cup. Then layer the sliced onions, carrots, and potatoes in that order, making sure to sprinkle a little salt and pepper over each layer. Follow this up by sprinkling the paprika and turmeric over the potatoes. Next, layer the sliced tomatoes over the potatoes. Place the fish over the tomatoes along with the chermoula it has been marinating in. Finish this by placing the sliced bell peppers, olives, and sliced lemons over the fish.
Cook the fish tagine: Pour ¼ cup of water into the tagine and place it on your stovetop over medium heat. Let the liquid in the tagine come to a simmer then cover it and reduce the heat to medium-low. Let the fish tagine cook for 45 minutes or until the fish and the vegetables are fully cooked and fork-tender.
Take the fish tagine off the heat and let it rest for a couple of minutes before serving, enjoy!