In a large saucepan of boiling salted water, cook the corn until just tender, about 1 minute. With a slotted spoon, transfer the corn to a bowl. Add the potatoes to the boiling water and cook until tender, about 6 minutes. Drain the potatoes and add to the bowl.
In a large pot, melt the butter. Add the fennel slices, leeks, onion, thyme and bay leaf and season with salt and pepper. Cover and cook over low heat, stirring occasionally, until the vegetables are softened, about 10 minutes. Add the milk, cream, corn and potatoes and simmer for about 3 minutes, smashing some of the potatoes to thicken the soup. Discard the thyme sprigs and bay leaf. Add the fish and simmer until heated through, about 2 minutes. Season lightly with salt and pepper. Ladle the chowder into bowls, garnish with fennel fronds and serve.