Here's how to clean your acrylic tub safely and naturally:
Wash your tub often, at least once a week, and use a mild soap -- plain old dishwashing liquid is best -- with lots of hot water. Always clean with a soft sponge. Never, ever use abrasive cleaning products (those containing fine grit; check the label) or abrasive cleaning pads. Waxing is another no-no. Your acrylic tub is naturally very shiny and designed to stay that way without additional help.
Lastly, is your water supply kind of alkaline? If so, you might notice a little hard-water buildup appearing over time. The easiest fix, believe it or not, is plain white vinegar. Spray a 50/50 mix of water and vinegar on the affected areas and let it soak for a while. Rub gently, then rinse. It's that simple!