What could you use to get brown stains off the surface of an acrylic snap-together tub?

If these stains are from rust, I would say try snow bowl. I know it sounds crazy, but it works! Put the snow bowl on ( try and get the thick formula so it doesn't run and actually sits on the stain) then let it sit for a few minutes(make sure not to leave it too long because it will start to eat the material). Make sure you have a window open or a fan on because the smell will be very strong and the chemicals are not good to inhale, and you may want to wear gloves because the strong chemicals will irritate the skin. Then just use a soft scrubber, like the green ones used in the kitchen and scrub it off.