Iron stained fiberglass

Wow.

I *wish* it was that easy. (Or course, there are plenty of pool stores that will sell you stuff that CLAIMS to make it that easy.)

But it's not. You need to:

1. Find out how the iron is getting into your pool, so you can prevent that.
2. Re-dissolve the iron (that's what stain removal agents do)
3. But NOT get algae while you do (stain removal agents aren't chlorine compatible!)
4. Restore your chlorine WITHOUT dropping the iron back out of the water (that it's dissolved in) and renewing your stains.
5. Removing the dissolved iron from the water.
6. Maintaining an ongoing process to prevent new stains by blocking, or removing, the iron that's entering your pool.

But, if you just want to get the stains off NOW, do this:

1. Buy some vitamin C tabs, and put a few on the steps. If the tabs lift the stains, it's iron and you can move to #2
2. Buy 2 lbs or so of vitamin C powder:
Vitamin C Crystals Ascorbic Acid 100% Pure Powder
Pure Ascorbic Acid (Vitamin C) Bulk Powder
3. Buy a couple of quarts of polyquat.
4. Add the polyquat, and let your chlorine drop to near zero.
5. Scatter the ascorbic acid on the steps.
6. Once the stains are gone, rechlorinate . . . at which time the stains are likely to re-appear.

Or, read about stain removal:
http://pool9.net/stains/