fiberglass pool reoccuring yellow sides

We've had our fiberglass inground pool for 11 years and this is the first time we've experienced this issue.
After opening the pool we had a really bad green algae problem which we were able clear in about a week.
Soon after that, we noticed the pool walls just under the waterline were a pale yellow color. Tried to wipe it off with a rag, and then very fine grit sandpaper with no success. It was almost like it was baked on. I went to the pool store and they advised a two-part treatment. One was a sequestering agent, and I think a metal-be-gone. I followed the instructions, and it worked. Pool looked great - for about a month, and then the yellow was back. Tried a different chemical recently, Super Stain Away. That one worked, too, but only for a week and the yellow is back.

We don't have well water; all our piping, except for the pool heater, is plastic. I've had several water tests since we opened the pool, and the only one that is out of whack is the amount of conditioner in the pool is high. We use granulated chlorine from Sam's Club, and shock the pool about every week with bags of shock.

Thanks for any advice you can give us!

Sounds like iron stains. You can check that very easily by getting some Vitamin C tabs; hold a handful underwater against a stained area for 3 minutes or so. If the Vitamin C (ascorbic acid) clears the stain, it's iron.

If it IS, you can remove the stains, BUT you MUST understand that removing the stains does NOT remove the iron from your pool water. You have to
1. Remove the stains, and put the iron BACK into the water.
2. KEEP the iron in the water, and then
3. REMOVE the iron FROM the water without re-staining the pool.

With just ONE exception, none of the pool products sold actually remove metals from your pool. Instead, they EITHER do #1 above (re-dissolve the metal, removing the stain) OR they do #2 (keep metals IN the water).

To actually REMOVE the metals you either have to do a sort of enhanced filtration or you have to use the CuLator product. There's a complete description of the process, here:
http://www.poolforum.com/pf2/showthr...acid-treatment

But, you first need to get a good test kit, here:

+ Here are links to the kits we recommend:

HTH 6-Way Test Kit @ Walmart
Taylor K2006A (3/4 oz bottles) @ Amazon
Taylor K2006C (2 oz bottles) @ Amazon