A tub is configured to contain a supply of water and be inserted into a chamber of a humidifier. The tub includes a tub base configured to contain a supply of water and a tub lid connected to the tub base. The tub lid includes an inlet configured to receive a flow of breathable gas to be humidified and an outlet for the humidified flow of breathable gas. The tub further includes a water level indicator configured to indicate a level of the supply of water in the tub base. The water level indicator includes an inclined portion configured to direct the flow of breathable gas from the inlet away from the outlet. The water level indicator may be visible through the inlet.