The ornamental design for a bathtub for sitz-baths, as shown and described.
1. Bath tub for sitz-baths
1.1 : Front
1.2 : Right
1.3 : Back
1.4 : Left
1.5 : Top
1.6 : Perspective
1.7 : Perspective
Reproduction 1.1 is a front view of the bath tub; reproduction 1.2 is a right view thereof; reproduction 1.3 is a back view thereof; reproduction 1.4 is a left view thereof; reproduction 1.5 is a top view thereof; reproduction 1.6 is a perspective top left view thereof; reproduction 1.7 is a perspective top right view thereof; bottom view is omitted because it is not viewable in normal use of the bath and includes no ornamentation.