I am contemplating a water change to reduce excessive CYA levels and also remove iron stains from the sides of my fiberglass pool.

I've read the sticky involving Ascorbic acid treatment with intention to dissolve existing stains then pump my pool while I simultaneously add water from my pond. I'm wondering about the addition of the sequestrant after the stains are dissolved. Since my goal is to remove as much of the water as possible is there any immediate need for HEDP? I can remove / add water at a rate of approx. 1500 gallons per hour and know I am actually only diluting the amount of dissolved metals and CYA.

I intend to run my water replacement scenario for approx 16 hours resulting in 24000 gallons of new water being pushed into a 25,000 gallon pool. I'm assuming there is minimal risk of re-stain during the pumping process since pond water PH checks 7.0 and I have no intention of adding chlorine at any time during the pumping process.

My plan is to add HEDP, chlorine and CYA if needed when the water swap is complete. Assuming my initial dose of ascorbic acid dissolves my stains is there any risk of stains reappearing while the pumping is in process and the ascorbic acid is diluted?

Is there anything else I should be concerned about?

Thanks in advance for any advice!