Heat oil in a 6-qt. Dutch oven over medium-high heat. Scrape marinade from chicken; reserve marinade. Add chicken to pot and cook, turning once, until browned, about 10 minutes. Transfer chicken to a plate and set aside. Add remaining green bell peppers, onions, and red bell peppers to pot, season with salt and pepper, and cook, stirring, until soft, 8–10 minutes. Add tomato paste and cook, stirring, for 1 minute. Return chicken to pot, along with reserved marinade and 1 cup water. Bring to a boil, reduce heat to medium-low, and cover pot slightly; cook, stirring occasionally, until chicken is tender, 25–30 minutes. Remove from heat and season with salt and pepper.
Rub fish evenly with half of the salt, turmeric, and cayenne; set aside. In a bowl, stir remaining salt, turmeric, and cayenne with powder and 1/2 cup water; set paste aside.
Heat oil in 12″ skillet over medium-high heat; add mustard seeds; cook for 1–2 minutes. Add cumin and fennel, cook for 30 seconds; add paste and chiles. Add fish, and cook, basting, until done, 10–12 minutes.