In a large stockpot, melt the butter over medium heat. Add the garlic and cook, stirring, for 30 seconds, keeping a close watch so it doesn’t burn. Add the onion, celery, bay leaf, thyme, and a pinch of salt, and cook, stirring, until softened, about 8 to 10 minutes. Add the potatoes and fish stock, and bring to a boil over medium-high heat. Reduce the heat to medium, and simmer until the potatoes are almost fork-tender, about 6 to 8 minutes. Stir in the milk, heavy cream, paprika, and lemon zest, season with salt and pepper, and bring the chowder back to a gentle simmer, stirring often. Add the fish and simmer for about 5 minutes, until the fish is cooked through and flakes easily with a fork. Taste and adjust the seasoning as necessary. Ladle into warmed bowls. Garnish with dill and chives, and serve right away, with lemon wedges alongside.