In a large stockpot, melt butter over medium-heat. Add the garlic and cook for 30 seconds, keeping a close watch that it doesn't burn. Add the onion, celery, bay leaf, thyme, and a pinch of salt, and sauté 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 (6-8 minutes).
Stir in the milk, heavy cream, smoked paprika, and lemon zest. Season with salt, and bring the chowder back to a simmer. Add the fish and simmer about 5 minutes until the fish is cooked through, opaque and easily flakes with a fork. Taste and adjust the seasonings. Divided the chowder into bowls, then garnish with fresh dill, chives and freshly ground black pepper.