Finely grate enough peel from 2 limes to measure 1 ½ teaspoons. Squeeze enough juice from 2 limes to measure 2 tablespoons. Cut third lime into wedges. In a small bowl, combine lime zest, lime juice, clam juice, fish sauce, sugar, curry powder and tumeric; set aside.
In a heavy large saucepan, heat canola oil over medium high heat. Saute the onions until translucent. Stir in minced garlic, ginger and Thai red curry paste and sauté until fragrant. Sprinkle with flour and stir in.
Whisk in coconut milk until well combined. Stir in clam juice mixture. Bring to a boil and then add the cubed sweet potatoes. Reduce heat and cook on a fast simmer until the sweet potatoes are tender, about 20 minutes.
While the pan is simmering and after the potatoes are fork-tender, carefully submerge the fish pieces into the coconut broth. Cook for about 15 minutes longer or until fish flakes easily with a fork. (Gently stir/move fish pieces around halfway through.)
Remove pan from heat and pour into a large serving dish. Sprinkle with ½ cup chopped cilantro and serve hot with cooked Jasmine rice, lime wedges and additional cilantro leaves, if desired.