When you shock the pool, you may get staining again. Whenever you have metal-contaminated water, it is critical that you never let your chlorine level get lower than the minimum advised in the Best Guess Chlorine Chart. That way, you won't have to shock for an impending algae bloom. Otherwise, you just have a repeating cycle. If you have algae, you have to get the chlorine level up enough to kill it and then, if you have staining, you'll have to do another ascorbic acid treatment.