I don't have a link for this directly, but apparently half a grapefruit and some large-grained sea salt is approximately the greatest cleaner for your bathtub ever. The salt gives it some "teeth" to scrub with, and the citrus breaks down goo and smells delightful. You can probably Google it and come up with how it's supposed to work, although I think "cut grapefruit in half, sprinkle generously with salt, attack the tub with it" is pretty straightforward. :) The author said that it did a good enough job that ze had had to let hir tub get dirty for a MONTH to get enough dirt to do reasonable before/after photos, so it's a lasting clean, too.