I have had a fiberglass pool with salt system and chlorinator for about 3 years now. The first year I had no problems whatsoever. Then I started getting stains on my pool which quickly disappear when I add ascorbic acid. My problem is that it happens about once a month and I don't know why. This has happened the past 2 summers. I've had my pool checked for metals and it shows very low amounts. In fact when I add the ascorbic acid I've tried putting metal free and I've tried not putting it in. The stain still comes back. I've tried to lower my PH and keep the chlorine level fairly low and the stain still comes back. No matter what I've tried, the stains still come back about every 3 - 4 weeks. It's kind of a pain because then I have to bring the chlorine level down to nothing, add the acid, filter for 24 hours and go about bringing the chlorine level back up again. My PH and alkalinity levels always remain pretty constant. Other than the stain, my pool is normally very easy to take care of. Is the stain returning that quickly something that's normal??? Am I doing something wrong?? Help!!