I also have a fiberglass pool, and I have learned to deal with the stains pretty effectively. When I get stains I use ascorbic acid, and a metal free product. The cloudy water is the metal free agent combining with the metal that has come back into solution. If you have a lot of metal in solution and the water is cloudy, you should not start to bring up your chlorine until the water is cleared - you can put a chlorine puck in the basket so the metals will stain the filter. You have to try and keep the ph around 7.2, not higher - it is hard to keep the ph there, because the stain treatment brings down the ph, plus the puck in the skimmer will add to lower ph. Be sure to keep your filter running 24/7 - once the water is clear, you can start to bring up the chlorine. I use regular bleach, and I don't run high chlorine levels - it isn't necessary especially if you keep your cya under 50. In my opinion, fiberglass pools tend to stain, I now treat it heavily at the beginning of the season with a metal free product, and keep the filter running 24/7 when I have high chlorine levels. All water and pools are different - you will have to experiment with yours to see where you need to keep your chlorine & ph to keep the stains away. Don't worry so much - if it stains, it can be taken care of!
16'x32' kidney 16K gal IG fiberglass pool; Bleach; Hayward 200lb sand filter; Hayward pump; 24hrs; Pf200; well; summer: none; winter: mesh; ; PF:7.5