I don't use anything but Scrubbing Bubbles,(but sometimes that won't remove the stains on the bottom.) I've used a magic eraser before and it worked really great in removing the stains.

I like Comet Bathroom Spray cleaner on my fiberglass shower stall floor. It uses 6% citric acid for cleaning soap scum and disinfecting. You need to follow the label directions, which call for leaving it on the stain for a couple minutes before wiping to remove stains.

And as southerngal25 says, a Mr. Clean magic eraser will remove stains as well.

Here is a link that might be useful: Comet bathroom spray cleaner

My jetted tub is fiberglass too, and when we bought it they said to use absolutely nothing abrasive, including Scrubbing Bubbles (who knew?). I love those magic erasers, but they fall into the abrasive category as well. I use a microfiber sponge with these little fingers on it--designed for washing cars I think. It grabs the crud right off without the need for cleansers. Once a week I soak the sponge in some Mr. Clean because it seems like I should, but it really doesn't do any better job than the sponge itself. Sponge lasts forever, had the same one for a year and clean the tub nightly. You get these in the car-washing section of discount stores.

I just moved in to a townhouse with all fiberglass tubs and what I find works best is to squirt Dawn or Palmolive in the tub and work it in and let it sit for a few minutes . I also used a toothbrush on the rough part that you step on and a lot of the stains came off. After I rinse it out I then spray Scrubbing Bubbles for an even whiter tub. Works for me! : )

I use scubbing bubbles too lets it nice & shinny too when done.

I take a pretty labor intensive methodology to my tubs... a nice paste of baking soda and dawn dish soap with a kitchen sponge (the green scrubby kind). I only get around to it every few months or so, and it takes about an hour to do a full top to bottom scrub down. Rinse with water, then spray with vinegar to remove any last baking soda residue. Rather than reaching over the edge, I find it easiest to just undress and scrub from inside the tub. Once it's all sparkly clean I reward myself for all my hard work with a nice hot bath!

Jules that is what use also, works for me. I don't have near the problem other people seem to have with cleaning maybe because it's a new home, not a rental and I never let it get dirty before I clean. It does get dusty though.