Light brownish rust like on pool liner below

[pectin232]

When I use Ascorbic Acid [Vitamin C] it works well to remove the stains. ..(just opened pool a few weeks ago). Problem is why did the stains happen and during the summer months how do I prevent the pool from getting this type of brownish stains? As it is expensive to keep putting about 5-6 lbs of Ascorbic Acid to remove the stains every other month? If metals are coming in how? Is it the metal ladder we have in the inground pool? Advise? I put in culator... and I believe it works but that is also very expensive. Water chemistry is all totally balanced [ph, alkalinity, etc etc] by the pool company that opened it. I think they is maybe metal in the water and we use Natural Chemistry -Metal Free to remove dissolved metals in it along with Culator Metal Remover. What else can be done. When I use Ascorbic Acid it works nearly within 1hr to remove the stains on a 24,000 gallon pool.