Humidifier tub

FIG. 1 is a front perspective view of a humidifier tub for a respiratory apparatus according to our new ornamental design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is a bottom view thereof;

FIG. 6 is a left side view thereof; and,

FIG. 7 is a right side view thereof.

The broken lines of the humidifier tub represent environmental structure and do not form a part of the claimed design. The dotted lines of the humidifier tub represent the border between claimed and unclaimed subject matter and do not form a part of the claimed design.