Place the chickpeas, onion, carrots, zucchini, red bell pepper, garlic, diced tomatoes, and vegetable broth in the slow cooker.
Add the cumin, coriander, cinnamon, turmeric, paprika, salt, and black pepper. Stir everything together until the spices are evenly distributed.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours, until the vegetables are soft.
About 10 minutes before the tagine finishes cooking, place the couscous in a large bowl and pour the boiling water over it. Cover the bowl and let it sit for 5 minutes.
Fluff the couscous with a fork. Spoon the tagine over the couscous and serve right away.