The answer to 'my bathtub smells bad' is that a foul odor in the bathtub is usually caused by a buildup of dirt, grime, or mildew in the drain or on the surface of the tub. This can happen over time as soap scum, hair, and other debris accumulate. The smell may also be due to a clogged drain that is allowing stagnant water to sit in the tub.
Some common symptoms of a smelly bathtub include a persistent unpleasant odor, visible grime or mildew, and potentially slow draining water. To address the issue, you can try some DIY cleaning methods like using a baking soda and vinegar solution, or a commercial drain cleaner. However, if the smell persists or you notice significant clogging, it's best to call in a professional plumber to inspect the drain and tub and provide a more thorough cleaning or repair.