Can someone please tell me how much of Ascorbic Acid do I need to put in a 35K fiberglass pool?

The metal stain is all over the walls and floor.
I tried 5 foam cups around the pool but it seems not enough. Acid desolves very quickly and I started to see white streaks on the wall but overall the pool is still not white.

I have a DE filter EC-75 and it doesn't have a circulate option. It is always filtering. Is it ok? Or should I turn it off while I am doing it?

Thanks,
B.