Place bacon strips in a large stockpot over medium-high heat. Cook bacon until crisp, then remove to paper towels to drain. Drain grease from the pot, reserving about 1 tablespoon. Set heat to medium, and fry onions in the reserved bacon grease until tender, about 5 minutes.
Add potatoes, and fill pot with just enough water to cover them. Bring to a boil, then cook until potatoes are almost tender, about 5 minutes. Add fish pieces, evaporated milk, whole milk, and butter. Bring to a boil, then simmer over low heat for 30 minutes. Season with salt and pepper to taste. Ladle into bowls, and top with crumbled bacon pieces.
