Abstract: An animal water tank that is heated to prevent freezing. The tank has a bottom with at least one upwardly extending sidewall to define sides of the tank. A tunnel defining a watertight channel extends across the tank. The tunnel has a top, sides, and a bottom. Ends of the tunnel terminate at openings in sides of the tank. A burner assembly in the tunnel produces heat from a combustible fuel to heat the tunnel. Openings in sides of the tank are covered by fin plates having fins obliquely angled downward to form louvered openings. Access panels cover the fin plates and extend vertically up the sidewalls of the tank to form chimneys that allow combustion gasses from the burner to draft upwardly out of the tank. Insulation may be added to sides of the tank to retain heat in the water.