Summarize:

With a little creative application, a single lemon can serve as both cleaning solution and sponge. Cut the lemon down the middle widthwise. You can now grip it by the peel-side and use it as a handheld scrubber to treat yucky shower stains.  Pick out the seeds before you start scrubbing so they don’t end up at the bottom of your shower. The natural acids in lemon juice are safe enough for you to use them to clean your shower as often as you’d like without having to worry about wearing down the acrylic finish. You may also have success using another type of citrus, such as grapefruit. Work the exposed fruit over the acrylic surface. As you press the lemon into the acrylic, the juice will gradually squeeze out, breaking up hard water stains, mold and mineral deposits.  After you’ve worn the lemon down, simply grab the other half or cut a new one and keep going over the area until it’s spotless. Just as a precaution, avoid using lemon in conjunction with other green cleaning ingredients like sea salt. Even water-soluble abrasives may be enough to scratch up acrylic. Once you’ve given the shower a thorough scrubbing, give the lemon juice a little time to work its magic. Though mild, the acids that the juice contains will usually be enough to deal with the majority of commonplace stains. The longer you let the juice soak, the better the end result will be.  Lemon juice is especially useful for dissolving stains that result from exposure to hard water or chemicals. Citric acid will not only clean and disinfect, it will also help brighten up the acrylic and restore it to its original luster. Flush every surface of the shower with warm water, taking care to get rid of any lingering juice or pulp from the lemon. If necessary, go back over the acrylic one more time with a soft towel or sponge. When you’re done, your shower should be pristine and have a fresh, clean scent!
Slice a large lemon in half. Rub the lemon directly onto the surface of the shower. Allow the lemon juice to sit for 10-20 minutes. Rinse the shower clean.